STATISTICAL CONCENTRATIONS
COMPARISON
OF H E A V Y M E T A L
IN V A R I O U S L O U I S I A N A S E D I M E N T S
K E V I N D. W H I T E and M A R T Y E. T I T T L E B A U M
Dept of Civil Engineering, Louisiana State University
(Received July, 1983) Abstract. The use of statistical 't' tests were used to determine lead, zinc, and cadmium enrichment in various Louisiana sediments. Both absolute metal concentrations and trace metal/conservative metal concentration ratios were used in comparing sampled sites to a 110 m deep background core taken just off the mouth of the Mississippi River. Concentration ratios were used to reduce the effects of certain chemical and physical sediment characteristics on the quantity of metal contained in a given sediment. Results from the comparison of sample sites to the background reveal metal enrichment at several sites. The University Lake sampling sites exhibit both lead and zinc enrichment when using both the concentration alone and ratio methods of comparison. Additionally, cadmium enrichment is indicated in the sediments of University Lake when using only the ratio method of comparison. Several sampled sediments in and around the New Orleans metropolitan area exhibited lead and cadmium enrichment.
I. Introduction
Louisiana's coastal areas contain thousands of square kilometers of estuaries and are among the most productive private and commercial fish and wildlife habitat areas in the United States. Excessive heavy metal contamination of these ecologically sensitive estuaries could evoke serious consequences. As a group, trace metals are not usually eliminated from the aquatic ecosystems by natural processes, in contrast to most organic pollutants. This is due to their non-biodegradability. Both toxic and non-toxic heavy metals tend to accumulate in bottom sediments from which they may be released by various processes of remobilization. Often times, these metals can move up the biologic chain, thereby reaching humans, where they produce chronic and acute ailments (Forstner and Wittmann, 1979). Forstner and Wittmann (1979) claim that the determination of metal levels in sediments can play a key role in detecting sources of pollution in aquatic systems. Although sediment analyses do not represent the extent of intoxication, they can he employed on a semi-quantitative basis in comparative studies to trace the sources of pollution, such as illegal discharges by factories. Under favorable conditions, pollution sources can even be detected long after input has taken place. Furthermore, it is possible to determine the development of pollution intensity from dated sediment cores provided they contain fine-grained depositions, in which sorbed, precipitated, organically bonded metal concentrations are accumulated. One field of sedimentary investigation, which is particularly useful in the present context, is marked by the study of vertical profiles from fine grained lakes, impoundments, estuaries, and coastal basins. Sediment cores can provide a historical record of events occurring in the watershed of a particular water body and Environmental Monitoring and Assessment 4 (1984) 163-170. 9 1984 by D. Reidel Publishing Company.
0167-6369/84/0042-0163501.20.
164
K. D. WHITE AND M. E. TITTLEBAUM
enable a reasonable estimate of the background level and changes in input over an extended period of time. Current technologies and analytical techniques make the analysis of heavy metals in sediment a routine practice. However, the determination of whether significant contamination exists is an unresolved problem. Metal distribution in sediments is influenced greatly by several sediment characteristics, principally grain size and organic content. Because of these influences, dissimilar sediment types have vastly different thresholds of contamination. Currently, the acceptable method of determining heavy metal contamination in sediments involves the comparison of metal concentration values. However, in some cases, this method can be significantly influenced by a sediment's physical and chemical composition. Although concentrations alone have proved a valid means of identifying contamination at sites of extreme additions, various parameters including sediment type, particle size, and analytical methods make comparisons between sites difficult. Another method of analyzing in order to determine contamination uses metal pair concentration ratios alone or in conjunction with absolute concentration values. Several researchers have advocated using metal concentration ratios in order to reduce the effects of grain size, organic content, and overall sediment characteristics due to spacial differences (Kemp et al., 1976; Allan and Brunskill, 1976; Forstner, 1976). It is felt that ratios of the element under consideration to another element of little variability (e.g., an element with 'conservative behavior') can determine whether metal enrichment has taken place. These conservative elements, except under rare circumstances, are unlikely to have enhanced concentrations in sediments related to man-made activities and because of their relatively high concentrations, even enhancement by cultural activities makes them less sensitive to change. Trace metals, on the other hand, are naturally presented in small concentrations and are greatly affected by man-made influences. Thus, ratios of trace metals to conservative metals reveal geochemical imbalances due to elevated trace metal concentrations normally associated with manmade activities. Literature values of background metal concentrations were used to calculate metal pair ratios to be compared to the metal pair ratios of the Texaco deep core background (Kemp et al., 1976). Results of these comparisons indicated striking similarities. Lake Erie background ratios for lead/iron were 1.7, 1.2, 1.7, and 1.0 for several locations, while the same ratio for the Texaco deep core averaged 1.6. Similar results were also seen for zinc/iron and cadmium/iron ratios, thus indicating the applicability of the ratio method and the quality of the Texaco deep core as a background. This study attempts to determine the existence of heavy metal contamination in various Louisiana sediments. Statistical t-tests were used to compare both trace metal/conservative metal concentration ratios and absolute metal concentrations to a 110 m deep background sediment core.
STATISTICAL COMPARISON OF HEAVYMETAL CONCENTRATIONS
165
2. Sampling Program and Experimental Procedures The sampling program employed was concentrated in three distinct areas: (1) freshwater diversions of the Mississippi River near New Orleans, (2) a deep core from the current Mississippi River Delta, and (3) University Lake in Baton Rouge. The background sample consisted of a number of deep core samples taken from Main Pass Block 75 of the Gulf of Mexico, near the Main Pass of the Mississippi River mouth in a water depth of 10 m. Samples were taken from a single boring ranging from 0.2 to 110 m in depth from mudline and were taken by McClelland Engineering Co. for Texaco Oil and donated to LSU for research purposes (McClelland Engineers, Inc., 1981). These deep core samples represent background samples for the Mississippi River deltaic region. Element concentrations found in the deep core are reported in Table I. The first sampling area was located principally along the lower Mississippi River, in the general vicinity of New Orleans. This sampling area consisted of six sampling sites. Four of these sites are designed, in part, to inject fresh water and sediment from the Mississippi River, into the salt marsh to prevent land loss or as navigational channels for commercial and industrial traffic. The remaining two sites are urban drainage systems draining the city of New Orleans. Because the Mississippi River drains roughly a third of the continental United States and hundreds of various industries are permitted to discharge wastewater into the river, the diversion of Mississippi River water and sediment into sensitive coastal marsh areas is a significant pollution risk. The second sampling area was located in the City Park/University Lake System in Baton Rouge. In the late 1920's and early 1930's, cypress swamps in Baton Rouge were TABLE I
Concentration ofelementsinthesediments Core location
Sample Depth Wt number (m) (g)
Texaco 2001 deep core 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
0.24 2.0 5.8 6.5 7.6 12.2 16.4 20.8 22.5 29,0 31.7 38.0 41.5 60.0 61.5 72.0 104.0
2.042 2,052 2,005 1.973 1,988 2.039 1.964 1.992 1.981 2,095 1,960 2.021 2,047 2.024 2,011 1,995 1.994
Vol. (ml)
(ppm) (ppm) (ppm) (ppm) (ppm) (~o)
Pb
Zn
AI (~o)
63.9 73.3 72.7 63.8 97.3 83.8 74.9 83.8 75.1 75.9 70.3 79.7 77.3 76.9 80.2 76.0 96.0
55 53 55 59 58 50 63 50 48 56 63 54 53 56 50 55 50
1.0 1.0 0.9 1.2 1.2 0.8 1.2 0.9 0.7 0.8 1.0 0.7 0.8 0.8 0.7 0.9 0.8
86 77 91 86 81 87 94 92 90 108 112 101 88 99 90 98 70
Cd
2.8 2.6 2,8 2,9 2.7 2,7 2.9 2.7 2.8 3.4 3.4 3.3 3.1 3.3 3.0 3.0 3.0
Cr
60 48 60 59 52 52 58 54 55 68 66 62 58 71 60 63 58
Ni
29 25 28 32 27 27 29 29 28 33 32 33 29 32 27 30 21
Fe
3.4 2.9 3.1 3,4 3.2 2.9 3.2 3.4 3.1 4.0 3.4 3.6 3.7 3.5 3.4 3.5 4.8
166
K. D. W HI TE AND M. E. T I T T L E B A U M
logged and dammed to form the existing City Park Lake and University Lake. The watershed of these two lakes is approximately 525 ha and is comprised of residential areas, roads, freeways, and parkland (Knaus et al., 1977). For over 40 yr erosion from cultural activities has contributed sediment to the lake bottoms rendering them shallow. Four core samples were taken and analyzed from University Lake and were designated UL-1 through UL-4. With the exception of the background deep core, sediment core samples were taken in aluminum coring tubes at each sampling site. The cores taken were from 15 to 45 cm in depth, and were 5 cm in diameter. Once taken, the cores were frozen, intact, until analysis. For analysis, all cores were divided into incremental sections based on visible layering, or if layering was not evident, 7.6 cm increments were obtained. Each incremental section was dried at 100 ~C and ground to a composite powder, from which a representative sample of approximately 2 g was taken. Thus, each sample was a composite of an incremental section. The digestion procedure used to solubilize metals from the sediment was obtained from Ritter et al. (1976) and consisted of a complete digestion utilizing hydrofluoric and hydrochloric acids. Metal concentrations were then determined spectrophotometrically. Using the obtained metal concentration values, trace metal/conservative metal concentration ratios were determined for each incremental section analyzed. In all cases, iron was used as the conservative metal species because of its relative abundance in the earth's crust. It is thus less likely to be influenced by man's activities. Lead, zinc, and cadmium were chosen as trace metal species to be studied. 3. Results Basically, the statistical 't' test evaluates the null hypothesis that the mean of one set of data is equal to the mean of another set of data (H o:/~l =/~2). Incorporation of the variances of each set of data is fundamental to this procedure. Assumptions of the 't' test insist that all samples are random in nature and that the data sets are normally distributed. The statistical 't' test was then used to determine the existence of any significant concentration and/or ratio differences (p > 0.05) between each sample location and the background deep core sample. Table I lists the variations in element concentrations with depth in the background deep core. The results obtained from the application of the statistical 't' test are presented in Tables II-VII. Sites found to have metal concentrations or ratios significantly different from the background are actually rejecting the null hypothesis and are indicated in the tables. The mean standard deviation (SD), and the number of incremental samples (N) obtained from each site are also listed for each sample site. Using concentration values alone, four sampling sites contain lead concentrations significantly different from the background (Table I). The sites are Harvey, Inner Harbor, and two sites in University Lake (UL-2 and UL-3). The use of lead/iron concentration ratios similarly indicate ratios significantly different from the background
TABLE II Comparison of incremental composite lead concentrations at each site to the background incremental composite lead concentration Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
54.6 114.3 66.2 91.4 113.2 46.0 43.0 56.2 148.8 148.5 109.2
4.4 18.8 17.3 13.9 101.1 10.6 17.2 3.6 45.1 49.3 53.3
Significant difference from background
Yes No Yes No No No No Yes Yes No
TABLE III Comparison of incremental composite zinc concentrations at each site to the background incremental composite zinc concentration Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL-4
17 3 5 5 6 6 7 5 6 6 6
91.2 135.3 75.6 111.2 105.8 50.0 32.0 53.4 210.5 190.5 134.0
10.5 57.0 18.2 13.6 47.9 15.2 8.3 6.8 109.4 105.4 28.3
Significant difference from background
No Yes Yes No Yes Yes Yes Yes No Yes
TABLE IV Comparison of incremental composite cadmium concentrations at each site to the background incremental composite cadmium concentration Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
3.0 3.7 2.4 3.5 2.6 2.0 1.1 1.3 3.6 8.9 2.7
0.3 0.3 0.2 0.4 0.6 0.6 0.2 0.2 1.1 5.6 0.3
Significant difference from background
Yes Yes Yes No Yes Yes Yes No Yes No
TABLE V Comparison of lead/iron concentration ratios at each site to the background lead/iron concentration ratio Site
N
Mean
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
1.6 2.9 2.8 2.5 4.1 2.3 4.4 4.0 4.1 3.8 4.1
0.2 0.5 1.0 0.3 3.0 0.6 3.2 0.4 0.5 0.8 1.7
Significant difference from background
Yes No Yes No Yes No Yes Yes Yes Yes
TABLE VI Comparison of zinc/iron concentration ratios at each site to the background zinc/iron concentration ratio Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL-4
17 3 5 5 6 6 7 5 6 6 6
2.7 3.5 3.1 3.0 4.0 2.4 3.0 3.8 3.5 4.8 5.1
0.4 1.4 0.6 0.4 2.0 0.2 0.9 0.4 1.8 2.0 0.6
Significant difference from background
No No No No No No Yes Yes No Yes
TABLE VII Comparison of cadmium/iron concentration ratios at each site to the background cadmium/iron concentration ratio Site
N
Mean
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
0.087 0.095 0.099 0.095 0.093 0.098 0.105 0.096 0.097 0.244 0.104
0.008 0.005 0.010 0.006 0.008 0.006 0.018 0.008 0.009 0.176 0.004
Significant difference from background
No Yes Yes No Yes Yes No Yes No Yes
STATISTICAL COMPARISON OF HEAVY METAL CONCENTRATIONS
169
for the same four sites (Harvey, Inner Harbor, University Lake), as well as Shell Beach and the two remaining sites in University Lake (Table IV). Zinc concentrations alone are found to be significantly different from the background in all but three sampling sites (Table II). No significant zinc concentration differences between site and background could be determined for the Harvey, Violet, and UL-3 sites. It should be noted that although the Algiers, Shell Beach, Bayou St. John, and UL-1 sites were determined to have a zinc concentration significantly different from the background, the mean zinc concentration for these sites are less than the mean zinc background concentration. In using zinc/iron concentration ratios, only three sites in University Lake exhibited zinc/iron ratios significantly different from the background ratio (Table VI). Similarly, the three University Lake sites showing zinc/iron concentration ratios significantly different from the background are also the three University Lake sites showing zinc concentrations alone significantly different from the background. Table III illustrates that in all but three sites, cadmium concentrations are significantly different from the background cadmium concentration. The Violet, UL-2 and UL-4 sites are the only sites that show no significant difference between site and background cadmium concentrations. Again, four sample sites that exhibit significantly different cadmium concentrations from the background actually have mean cadmium concentrations less than the mean cadmium concentration of the background. These sites are Algiers, Shell Beach, Bayou St. John, and U L - 1. The use of cadmium/iron concentration ratios indicates that the Algiers, Inner Harbor, Shell Beach, Bayou St. John, UL-2 and UL--4 sites all show significantly different cadmium/iron ratios than the background (Table VII). 4. Discussion and Conclusions
Based on the results of the statistical 't' tests, comparing sample sites to a 110 m deep background core sample, several insights into the degree of metal enrichment in Louisiana sediments have been gained. Both concentration values and metal pair ratios indicate that lead, zinc, and cadmium are present in various Louisiana sediments at significantly higher levels than are found in a background sediment. This indicates that a relative enrichment of these three metals has occurred in the various locations at some point in the past. The enrichment of lead in the sediments of University Lake is substantiated by statistical 't' test results. All four University Lake sites show significantly different lead/iron concentration ratios than are found in the background, while two of the lake sites show lead concentrations alone significantly different from the background. The Harvey and Inner Harbor sites also exhibit a presence of lead significantly different from the background when using both the concentration and the ratio methods of analysis. The presence of zinc at levels significantly different from the background occur in three of the four University Lake sites when employing both the concentration and ratio methods of analysis. The use of concentration values alone show zinc levels in the
170
K. D. WHITE AND M. E. TITTLEBAUM
Algiers, Inner Harbor, Shell Beach, and Bayou St. John sites are significantly different from the background. These differences are not supported when using concentration ratios. Only the Inner Harbor site exhibited significant cadmium enrichment when using both the concentration and the ratio methods of analysis. However, significant cadmium enrichment was indicated in the Algiers, Shell Beach, Bayou St. John, and two University Lake sites, as well as Inner Harbor, when using the cadmium/iron ratio method of analysis. Because of the quality of the deep core and the fact that metal concentration ratios have been adopted in order to reduce the effects of certain physical and chemical sediment characteristics, it is felt that the use of metal concentration ratios is a more reliable method to determine metal contamination. Specifically, the effects of sediment grain size and organic content on the quantity of metal 'held' by a sediment are reduced when the use of metal concentration ratios are employed. Thus, a better representation of actual metal content is revealed through the use of concentration ratios. It is the conclusion of this research that heavy metal enrichment, in the form of lead, zinc, and cadmium, has occurred in the sediments of the University Lake at some point in the past. Additionally, the Inner Harbor site exhibited lead and cadmium enrichment when using both analysis procedures. It was also determined that significant lead enrichment has occurred in the sediments of the Harvey site.
References Allan, R. J. and Brunskill, G. J.: 1976, 'Relative Atomic Variation (RAV) of ELements in Lake Sediments: Lake Winnipeg and other Canadian Lakes', in Interactions Between Sediments and Freshwater. Forstner, U.: 1976, 'Metal Concentrations in Freshwater Sediments - Natural Background and Cultural Effects', in Interactions Between Sediments and Freshwater. Forstner, N. and Wittmann, C. T. W.: 1979, Metal Pollution in Aquatic Environment. Kemp, A. L. W., Thomas, R. L., Dell, C. I., and Jaquet, J. M.: 1976, 'Cultural Impact on the Geochemistry of Sediments in Lake Erie', J. Fish. Res. Board Canada 33. Knaus, R., Addision, W. W., Arman, A., Bryan, C. F., Gambrell, R. P., and Huner, J. V.: 1977, 'Lakes Restoration Project'. McClelland Engineers, Inc., Geotechnical Consultants, New Orleans, Louisiana: 1981, 'Geotechnical Investigation Block 75, Main Pass Area'. Ritter, G. J. et al.: 1976, 'Comparisons of Sample Preparation Techniques for Atomic Absorptiorl AH~dysis of Sewage Sludge and Soil', Atomic Absorption Newsletter 15, No. 1.
COMPARISON
OF H E A V Y M E T A L
IN V A R I O U S L O U I S I A N A S E D I M E N T S
K E V I N D. W H I T E and M A R T Y E. T I T T L E B A U M
Dept of Civil Engineering, Louisiana State University
(Received July, 1983) Abstract. The use of statistical 't' tests were used to determine lead, zinc, and cadmium enrichment in various Louisiana sediments. Both absolute metal concentrations and trace metal/conservative metal concentration ratios were used in comparing sampled sites to a 110 m deep background core taken just off the mouth of the Mississippi River. Concentration ratios were used to reduce the effects of certain chemical and physical sediment characteristics on the quantity of metal contained in a given sediment. Results from the comparison of sample sites to the background reveal metal enrichment at several sites. The University Lake sampling sites exhibit both lead and zinc enrichment when using both the concentration alone and ratio methods of comparison. Additionally, cadmium enrichment is indicated in the sediments of University Lake when using only the ratio method of comparison. Several sampled sediments in and around the New Orleans metropolitan area exhibited lead and cadmium enrichment.
I. Introduction
Louisiana's coastal areas contain thousands of square kilometers of estuaries and are among the most productive private and commercial fish and wildlife habitat areas in the United States. Excessive heavy metal contamination of these ecologically sensitive estuaries could evoke serious consequences. As a group, trace metals are not usually eliminated from the aquatic ecosystems by natural processes, in contrast to most organic pollutants. This is due to their non-biodegradability. Both toxic and non-toxic heavy metals tend to accumulate in bottom sediments from which they may be released by various processes of remobilization. Often times, these metals can move up the biologic chain, thereby reaching humans, where they produce chronic and acute ailments (Forstner and Wittmann, 1979). Forstner and Wittmann (1979) claim that the determination of metal levels in sediments can play a key role in detecting sources of pollution in aquatic systems. Although sediment analyses do not represent the extent of intoxication, they can he employed on a semi-quantitative basis in comparative studies to trace the sources of pollution, such as illegal discharges by factories. Under favorable conditions, pollution sources can even be detected long after input has taken place. Furthermore, it is possible to determine the development of pollution intensity from dated sediment cores provided they contain fine-grained depositions, in which sorbed, precipitated, organically bonded metal concentrations are accumulated. One field of sedimentary investigation, which is particularly useful in the present context, is marked by the study of vertical profiles from fine grained lakes, impoundments, estuaries, and coastal basins. Sediment cores can provide a historical record of events occurring in the watershed of a particular water body and Environmental Monitoring and Assessment 4 (1984) 163-170. 9 1984 by D. Reidel Publishing Company.
0167-6369/84/0042-0163501.20.
164
K. D. WHITE AND M. E. TITTLEBAUM
enable a reasonable estimate of the background level and changes in input over an extended period of time. Current technologies and analytical techniques make the analysis of heavy metals in sediment a routine practice. However, the determination of whether significant contamination exists is an unresolved problem. Metal distribution in sediments is influenced greatly by several sediment characteristics, principally grain size and organic content. Because of these influences, dissimilar sediment types have vastly different thresholds of contamination. Currently, the acceptable method of determining heavy metal contamination in sediments involves the comparison of metal concentration values. However, in some cases, this method can be significantly influenced by a sediment's physical and chemical composition. Although concentrations alone have proved a valid means of identifying contamination at sites of extreme additions, various parameters including sediment type, particle size, and analytical methods make comparisons between sites difficult. Another method of analyzing in order to determine contamination uses metal pair concentration ratios alone or in conjunction with absolute concentration values. Several researchers have advocated using metal concentration ratios in order to reduce the effects of grain size, organic content, and overall sediment characteristics due to spacial differences (Kemp et al., 1976; Allan and Brunskill, 1976; Forstner, 1976). It is felt that ratios of the element under consideration to another element of little variability (e.g., an element with 'conservative behavior') can determine whether metal enrichment has taken place. These conservative elements, except under rare circumstances, are unlikely to have enhanced concentrations in sediments related to man-made activities and because of their relatively high concentrations, even enhancement by cultural activities makes them less sensitive to change. Trace metals, on the other hand, are naturally presented in small concentrations and are greatly affected by man-made influences. Thus, ratios of trace metals to conservative metals reveal geochemical imbalances due to elevated trace metal concentrations normally associated with manmade activities. Literature values of background metal concentrations were used to calculate metal pair ratios to be compared to the metal pair ratios of the Texaco deep core background (Kemp et al., 1976). Results of these comparisons indicated striking similarities. Lake Erie background ratios for lead/iron were 1.7, 1.2, 1.7, and 1.0 for several locations, while the same ratio for the Texaco deep core averaged 1.6. Similar results were also seen for zinc/iron and cadmium/iron ratios, thus indicating the applicability of the ratio method and the quality of the Texaco deep core as a background. This study attempts to determine the existence of heavy metal contamination in various Louisiana sediments. Statistical t-tests were used to compare both trace metal/conservative metal concentration ratios and absolute metal concentrations to a 110 m deep background sediment core.
STATISTICAL COMPARISON OF HEAVYMETAL CONCENTRATIONS
165
2. Sampling Program and Experimental Procedures The sampling program employed was concentrated in three distinct areas: (1) freshwater diversions of the Mississippi River near New Orleans, (2) a deep core from the current Mississippi River Delta, and (3) University Lake in Baton Rouge. The background sample consisted of a number of deep core samples taken from Main Pass Block 75 of the Gulf of Mexico, near the Main Pass of the Mississippi River mouth in a water depth of 10 m. Samples were taken from a single boring ranging from 0.2 to 110 m in depth from mudline and were taken by McClelland Engineering Co. for Texaco Oil and donated to LSU for research purposes (McClelland Engineers, Inc., 1981). These deep core samples represent background samples for the Mississippi River deltaic region. Element concentrations found in the deep core are reported in Table I. The first sampling area was located principally along the lower Mississippi River, in the general vicinity of New Orleans. This sampling area consisted of six sampling sites. Four of these sites are designed, in part, to inject fresh water and sediment from the Mississippi River, into the salt marsh to prevent land loss or as navigational channels for commercial and industrial traffic. The remaining two sites are urban drainage systems draining the city of New Orleans. Because the Mississippi River drains roughly a third of the continental United States and hundreds of various industries are permitted to discharge wastewater into the river, the diversion of Mississippi River water and sediment into sensitive coastal marsh areas is a significant pollution risk. The second sampling area was located in the City Park/University Lake System in Baton Rouge. In the late 1920's and early 1930's, cypress swamps in Baton Rouge were TABLE I
Concentration ofelementsinthesediments Core location
Sample Depth Wt number (m) (g)
Texaco 2001 deep core 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
0.24 2.0 5.8 6.5 7.6 12.2 16.4 20.8 22.5 29,0 31.7 38.0 41.5 60.0 61.5 72.0 104.0
2.042 2,052 2,005 1.973 1,988 2.039 1.964 1.992 1.981 2,095 1,960 2.021 2,047 2.024 2,011 1,995 1.994
Vol. (ml)
(ppm) (ppm) (ppm) (ppm) (ppm) (~o)
Pb
Zn
AI (~o)
63.9 73.3 72.7 63.8 97.3 83.8 74.9 83.8 75.1 75.9 70.3 79.7 77.3 76.9 80.2 76.0 96.0
55 53 55 59 58 50 63 50 48 56 63 54 53 56 50 55 50
1.0 1.0 0.9 1.2 1.2 0.8 1.2 0.9 0.7 0.8 1.0 0.7 0.8 0.8 0.7 0.9 0.8
86 77 91 86 81 87 94 92 90 108 112 101 88 99 90 98 70
Cd
2.8 2.6 2,8 2,9 2.7 2,7 2.9 2.7 2.8 3.4 3.4 3.3 3.1 3.3 3.0 3.0 3.0
Cr
60 48 60 59 52 52 58 54 55 68 66 62 58 71 60 63 58
Ni
29 25 28 32 27 27 29 29 28 33 32 33 29 32 27 30 21
Fe
3.4 2.9 3.1 3,4 3.2 2.9 3.2 3.4 3.1 4.0 3.4 3.6 3.7 3.5 3.4 3.5 4.8
166
K. D. W HI TE AND M. E. T I T T L E B A U M
logged and dammed to form the existing City Park Lake and University Lake. The watershed of these two lakes is approximately 525 ha and is comprised of residential areas, roads, freeways, and parkland (Knaus et al., 1977). For over 40 yr erosion from cultural activities has contributed sediment to the lake bottoms rendering them shallow. Four core samples were taken and analyzed from University Lake and were designated UL-1 through UL-4. With the exception of the background deep core, sediment core samples were taken in aluminum coring tubes at each sampling site. The cores taken were from 15 to 45 cm in depth, and were 5 cm in diameter. Once taken, the cores were frozen, intact, until analysis. For analysis, all cores were divided into incremental sections based on visible layering, or if layering was not evident, 7.6 cm increments were obtained. Each incremental section was dried at 100 ~C and ground to a composite powder, from which a representative sample of approximately 2 g was taken. Thus, each sample was a composite of an incremental section. The digestion procedure used to solubilize metals from the sediment was obtained from Ritter et al. (1976) and consisted of a complete digestion utilizing hydrofluoric and hydrochloric acids. Metal concentrations were then determined spectrophotometrically. Using the obtained metal concentration values, trace metal/conservative metal concentration ratios were determined for each incremental section analyzed. In all cases, iron was used as the conservative metal species because of its relative abundance in the earth's crust. It is thus less likely to be influenced by man's activities. Lead, zinc, and cadmium were chosen as trace metal species to be studied. 3. Results Basically, the statistical 't' test evaluates the null hypothesis that the mean of one set of data is equal to the mean of another set of data (H o:/~l =/~2). Incorporation of the variances of each set of data is fundamental to this procedure. Assumptions of the 't' test insist that all samples are random in nature and that the data sets are normally distributed. The statistical 't' test was then used to determine the existence of any significant concentration and/or ratio differences (p > 0.05) between each sample location and the background deep core sample. Table I lists the variations in element concentrations with depth in the background deep core. The results obtained from the application of the statistical 't' test are presented in Tables II-VII. Sites found to have metal concentrations or ratios significantly different from the background are actually rejecting the null hypothesis and are indicated in the tables. The mean standard deviation (SD), and the number of incremental samples (N) obtained from each site are also listed for each sample site. Using concentration values alone, four sampling sites contain lead concentrations significantly different from the background (Table I). The sites are Harvey, Inner Harbor, and two sites in University Lake (UL-2 and UL-3). The use of lead/iron concentration ratios similarly indicate ratios significantly different from the background
TABLE II Comparison of incremental composite lead concentrations at each site to the background incremental composite lead concentration Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
54.6 114.3 66.2 91.4 113.2 46.0 43.0 56.2 148.8 148.5 109.2
4.4 18.8 17.3 13.9 101.1 10.6 17.2 3.6 45.1 49.3 53.3
Significant difference from background
Yes No Yes No No No No Yes Yes No
TABLE III Comparison of incremental composite zinc concentrations at each site to the background incremental composite zinc concentration Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL-4
17 3 5 5 6 6 7 5 6 6 6
91.2 135.3 75.6 111.2 105.8 50.0 32.0 53.4 210.5 190.5 134.0
10.5 57.0 18.2 13.6 47.9 15.2 8.3 6.8 109.4 105.4 28.3
Significant difference from background
No Yes Yes No Yes Yes Yes Yes No Yes
TABLE IV Comparison of incremental composite cadmium concentrations at each site to the background incremental composite cadmium concentration Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
3.0 3.7 2.4 3.5 2.6 2.0 1.1 1.3 3.6 8.9 2.7
0.3 0.3 0.2 0.4 0.6 0.6 0.2 0.2 1.1 5.6 0.3
Significant difference from background
Yes Yes Yes No Yes Yes Yes No Yes No
TABLE V Comparison of lead/iron concentration ratios at each site to the background lead/iron concentration ratio Site
N
Mean
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
1.6 2.9 2.8 2.5 4.1 2.3 4.4 4.0 4.1 3.8 4.1
0.2 0.5 1.0 0.3 3.0 0.6 3.2 0.4 0.5 0.8 1.7
Significant difference from background
Yes No Yes No Yes No Yes Yes Yes Yes
TABLE VI Comparison of zinc/iron concentration ratios at each site to the background zinc/iron concentration ratio Site
N
Mean (ppm)
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL-4
17 3 5 5 6 6 7 5 6 6 6
2.7 3.5 3.1 3.0 4.0 2.4 3.0 3.8 3.5 4.8 5.1
0.4 1.4 0.6 0.4 2.0 0.2 0.9 0.4 1.8 2.0 0.6
Significant difference from background
No No No No No No Yes Yes No Yes
TABLE VII Comparison of cadmium/iron concentration ratios at each site to the background cadmium/iron concentration ratio Site
N
Mean
SD
Background Harvey Algiers Inner Harbor Violet Shell Beach Bayou St. John UL-1 UL-2 UL-3 UL--4
17 3 5 5 6 6 7 5 6 6 6
0.087 0.095 0.099 0.095 0.093 0.098 0.105 0.096 0.097 0.244 0.104
0.008 0.005 0.010 0.006 0.008 0.006 0.018 0.008 0.009 0.176 0.004
Significant difference from background
No Yes Yes No Yes Yes No Yes No Yes
STATISTICAL COMPARISON OF HEAVY METAL CONCENTRATIONS
169
for the same four sites (Harvey, Inner Harbor, University Lake), as well as Shell Beach and the two remaining sites in University Lake (Table IV). Zinc concentrations alone are found to be significantly different from the background in all but three sampling sites (Table II). No significant zinc concentration differences between site and background could be determined for the Harvey, Violet, and UL-3 sites. It should be noted that although the Algiers, Shell Beach, Bayou St. John, and UL-1 sites were determined to have a zinc concentration significantly different from the background, the mean zinc concentration for these sites are less than the mean zinc background concentration. In using zinc/iron concentration ratios, only three sites in University Lake exhibited zinc/iron ratios significantly different from the background ratio (Table VI). Similarly, the three University Lake sites showing zinc/iron concentration ratios significantly different from the background are also the three University Lake sites showing zinc concentrations alone significantly different from the background. Table III illustrates that in all but three sites, cadmium concentrations are significantly different from the background cadmium concentration. The Violet, UL-2 and UL-4 sites are the only sites that show no significant difference between site and background cadmium concentrations. Again, four sample sites that exhibit significantly different cadmium concentrations from the background actually have mean cadmium concentrations less than the mean cadmium concentration of the background. These sites are Algiers, Shell Beach, Bayou St. John, and U L - 1. The use of cadmium/iron concentration ratios indicates that the Algiers, Inner Harbor, Shell Beach, Bayou St. John, UL-2 and UL--4 sites all show significantly different cadmium/iron ratios than the background (Table VII). 4. Discussion and Conclusions
Based on the results of the statistical 't' tests, comparing sample sites to a 110 m deep background core sample, several insights into the degree of metal enrichment in Louisiana sediments have been gained. Both concentration values and metal pair ratios indicate that lead, zinc, and cadmium are present in various Louisiana sediments at significantly higher levels than are found in a background sediment. This indicates that a relative enrichment of these three metals has occurred in the various locations at some point in the past. The enrichment of lead in the sediments of University Lake is substantiated by statistical 't' test results. All four University Lake sites show significantly different lead/iron concentration ratios than are found in the background, while two of the lake sites show lead concentrations alone significantly different from the background. The Harvey and Inner Harbor sites also exhibit a presence of lead significantly different from the background when using both the concentration and the ratio methods of analysis. The presence of zinc at levels significantly different from the background occur in three of the four University Lake sites when employing both the concentration and ratio methods of analysis. The use of concentration values alone show zinc levels in the
170
K. D. WHITE AND M. E. TITTLEBAUM
Algiers, Inner Harbor, Shell Beach, and Bayou St. John sites are significantly different from the background. These differences are not supported when using concentration ratios. Only the Inner Harbor site exhibited significant cadmium enrichment when using both the concentration and the ratio methods of analysis. However, significant cadmium enrichment was indicated in the Algiers, Shell Beach, Bayou St. John, and two University Lake sites, as well as Inner Harbor, when using the cadmium/iron ratio method of analysis. Because of the quality of the deep core and the fact that metal concentration ratios have been adopted in order to reduce the effects of certain physical and chemical sediment characteristics, it is felt that the use of metal concentration ratios is a more reliable method to determine metal contamination. Specifically, the effects of sediment grain size and organic content on the quantity of metal 'held' by a sediment are reduced when the use of metal concentration ratios are employed. Thus, a better representation of actual metal content is revealed through the use of concentration ratios. It is the conclusion of this research that heavy metal enrichment, in the form of lead, zinc, and cadmium, has occurred in the sediments of the University Lake at some point in the past. Additionally, the Inner Harbor site exhibited lead and cadmium enrichment when using both analysis procedures. It was also determined that significant lead enrichment has occurred in the sediments of the Harvey site.
References Allan, R. J. and Brunskill, G. J.: 1976, 'Relative Atomic Variation (RAV) of ELements in Lake Sediments: Lake Winnipeg and other Canadian Lakes', in Interactions Between Sediments and Freshwater. Forstner, U.: 1976, 'Metal Concentrations in Freshwater Sediments - Natural Background and Cultural Effects', in Interactions Between Sediments and Freshwater. Forstner, N. and Wittmann, C. T. W.: 1979, Metal Pollution in Aquatic Environment. Kemp, A. L. W., Thomas, R. L., Dell, C. I., and Jaquet, J. M.: 1976, 'Cultural Impact on the Geochemistry of Sediments in Lake Erie', J. Fish. Res. Board Canada 33. Knaus, R., Addision, W. W., Arman, A., Bryan, C. F., Gambrell, R. P., and Huner, J. V.: 1977, 'Lakes Restoration Project'. McClelland Engineers, Inc., Geotechnical Consultants, New Orleans, Louisiana: 1981, 'Geotechnical Investigation Block 75, Main Pass Area'. Ritter, G. J. et al.: 1976, 'Comparisons of Sample Preparation Techniques for Atomic Absorptiorl AH~dysis of Sewage Sludge and Soil', Atomic Absorption Newsletter 15, No. 1.
