EXPLANATION 114/15 Number of samples/Number of samples Highest detection limit with concentrations below the laboratory Maximum Contaminant Level (mandatory) (U.S. Environmental reporting limit Protection Agency, 1994a) Outlier data value more than 3 times the interquartile range outside the quartile Outlier data value less than or equal to 3 and more than 1.5 times the interquartile range outside the quartile Data value less than or equal to 1.5 times the interquartile range outside the quartile 75th percentile Median 25th percentile Single sample Figure 22. Boxplots of concentrations of selected nutrients for selected aquifers.--Continued Trace Elements in the Precambrian, Deadwood, Madison, Minnelusa, and Minnekahta aquifers than in the other aquifers. Summary statistics for selected trace elements, The Sundance aquifer generally has the highest sele- including aluminum, arsenic, barium, boron, cadmium, nium concentrations of all aquifers considered in this chromium, cobalt, copper, iron, lead, lithium, manga- report. In general, strontium concentrations are lower nese, mercury, molybdenum, nickel, selenium, silver, and have smaller variability in the Precambrian, strontium, vanadium, and zinc, are presented in table 7. Deadwood, Madison, and Minnekahta aquifers than in The significance of the various trace elements is the other aquifers. described in table 1. Boxplots are presented in figure 23 for each of the trace elements. Some of the trace elements, such as aluminum, cadmium, chro- Precambrian Aquifers mium, copper, iron, lead, mercury, nickel, and zinc, are Concentrations of several trace elements in particularly susceptible to sampling/processing con- samples collected from the Precambrian aquifers tamination. Because ultra-clean techniques were not exceed various drinking water standards. One of 51 used in sample collection, some of the results presented samples exceeds the lower value of the SMCL range of may reflect sample contamination. 50 to 200 µg/L for aluminum. Of the 52 samples In all aquifers considered in this report, stron- analyzed for arsenic concentrations, one sample tium generally is higher in concentration than the other exceeds the current MCL of 50 µg/L for arsenic, and trace elements. Barium, boron, iron, manganese, four samples exceed the proposed MCL of 10 µg/L for lithium, and zinc concentrations also may be high in arsenic. About 15 percent of the samples (14 of 91 comparison to other trace elements. Concentrations samples) exceed the SMCL of 300 µg/L for iron, and and variability of many trace elements are small in the 35 percent of the samples (33 of 93 samples) exceed aquifers. Boron concentrations generally are much the SMCL of 50 µg/L for manganese. higher and have larger variability in the minor aquifers Of all samples collected from the major aquifers, than in the major aquifers, with generally higher con- samples from the Precambrian aquifers have the centrations in the Inyan Kara aquifer than the other highest mean manganese concentration and the highest major aquifers. The Inyan Kara and Precambrian median zinc concentration. Samples from the Precam- aquifers generally have lower barium concentrations brian aquifers also have the lowest mean and median and generally higher manganese concentrations than concentrations of boron (equal to the median concen- the other major aquifers. Lithium concentrations tration of the Madison aquifer) and strontium and the generally are much lower and have smaller variability lowest median iron concentration. Water-Quality Characteristics of Selected Aquifers 75 Table 7. Summary of concentrations of trace elements in ground water [Results based on data stored in U.S. Geological Survey National Water Information System water-quality database. Results in micrograms per liter. One microgram per liter is approximately equal to one part per billion; --, not analyzed or not determined; <, less than indicated detection limit] Number of Number of Dissolved constituent censored Mean Median Minimum Maximum samples samples Precambrian aquifers Aluminum 51 39 6.5 3.7 <10 54 Arsenic 52 24 4.2 1.2 <0.5 103 Barium 52 1 33 27 <2.0 120 Boron 60 0 29 20 6.0 227 Cadmium 1 0 -- -- 3.5 3.5 Chromium 52 45 1-- 1-- <4.0 13 Cobalt 51 37 1.5 1.0 <2.0 7.0 Copper 58 37 30 0.5 <2.0 517 Iron 91 33 267 11 <10 11,000 Lead 1 0 -- -- 10 10 Lithium 51 0 17 9.0 3.0 113 Manganese 93 31 136 10 <2.0 1,100 Mercury 0 -- -- -- -- -- Molybdenum 51 31 5.0 3.5 <4.0 22 Nickel 51 42 1-- 1-- <4.0 17 Selenium 52 18 0.3 0.3 <0.2 0.7 Silver 52 37 1.4 1.1 <2.0 6.0 Strontium 51 0 170 140 11 619 Vanadium 51 37 4.1 2.0 <4.0 37 Zinc 58 0 168 67 4.0 3,236 Deadwood aquifer Aluminum 6 4 -- -- <10 40 Arsenic 11 2 2.9 1.3 <1.0 11 Barium 17 0 234 100 14 1,500 Boron 9 0672511290 Cadmium 4 3 -- -- <1.0 1.0 Chromium 6 4 -- -- 1.0 6.0 Cobalt 5 3 -- -- <2.0 7.8 Copper 10 6 20 1.7 <1.0 184 Iron 34 1 371 80 9.0 2,500 Lead 4 2 -- -- <1.0 2.0 Lithium 7 1 31 9.0 <2.0 140 Manganese 34 10 31 7.0 <1.0 340 Mercury 6 6 -- -- <0.1 <0.1 Molybdenum 7 4 6.1 3.1 <1.0 16.0 Nickel 4 3 -- -- <4.0 6.0 Selenium 10 5 0.4 0.2 <0.2 1.0 Silver 5 3 -- -- <1.0 3.0 Strontium 7 0 442 449 54 1,161 Vanadium 5 3 -- -- <4.0 27 Zinc 11 3 279 40 <3.0 2,430 76 Water-Quality Characteristics in the Black Hills Area, South Dakota Table 7. Summary of concentrations of trace elements in ground water—Continued [Results based on data stored in U.S. Geological Survey National Water Information System water-quality database. Results in micrograms per liter. One microgram per liter is approximately equal to one part per billion; --, not analyzed or not determined; <, less than indicated detection limit] Number of Number of Dissolved constituent censored Mean Median Minimum Maximum samples samples Madison aquifer Aluminum 26 16 15 4.9 <10 100 Arsenic 60 7 3.6 3.0 <0.5 27 Barium 37 0 85 51 11 300 Boron 63 5 59 20 9.0 588 Cadmium 38 35 -- -- <0.1 10 Chromium 33 32 -- -- <1.0 6.0 Cobalt 32 22 1.8 1.3 <2.0 7.0 Copper 65 46 4.5 0.6 <1.0 134 Iron 101 29 195 20 <3.0 7,400 Lead 34 24 2.2 0.9 <1.0 29 Lithium 41 9 25 7.0 2.0 236 Manganese 99 47 31 2.7 0.6 710 Mercury 35 30 1-- 1-- <0.1 0.3 Molybdenum 39 21 5.7 2.5 <1.0 34 Nickel 19 17 -- -- <4.0 10 Selenium 55 14 1.9 0.7 <0.2 18 Silver 48 42 1-- 1-- <0.2 4.0 Strontium 45 0 501 225 60 3,300 Vanadium 37 26 2.7 2.0 0.7 12 Zinc 72 15 100 13 2.0 1,407 Minnelusa aquifer Aluminum 41 31 23 0.7 <10 400 Arsenic 67 11 4.9 2.3 <0.5 30 Barium 57 0 103 68 2.0 400 Boron 101351306.0340 Cadmium 28 26 -- -- <1.0 3.0 Chromium 48 37 2.5 1.3 <1.0 10 Cobalt 46 28 2.0 1.5 <2.0 8.0 Copper 85 56 15 1.1 <1.0 670 Iron 197 59 199 20 <0.1 3,700 Lead 21 17 1-- 1-- <1.0 30 Lithium 57 1 21 10 4.0 160 Manganese 166 83 70 5.1 <1.0 7,200 Mercury 25 24 -- -- <0.1 0.2 Molybdenum 55 31 8.2 3.0 <1.0 102 Nickel 37 24 3.4 2.8 <2.0 10 Selenium 67 9 1.4 0.5 <0.2 12 Silver 51 36 1.5 1.3 <1.0 5.0 Strontium 57 0 1,547 409 62 11,000 Vanadium 53 18 6.9 5.0 0.7 30 Zinc 96 11 272 33 <3.0 10,000 Water-Quality Characteristics of Selected Aquifers 77 Table 7. Summary of concentrations of trace elements in ground water—Continued [Results based on data stored in U.S. Geological Survey National Water Information System water-quality database. Results in micrograms per liter. One microgram per liter is approximately equal to one part per billion; --, not analyzed or not determined; <, less than indicated detection limit] Number of Number of Dissolved constituent censored Mean Median Minimum Maximum samples samples Minnekahta aquifer Aluminum 1 1 -- -- <10 <10 Arsenic 4 1 2.4 2.1 <1.0 5.0 Barium 4 0 148 125 42 300 Boron 7 0 118 40 19 620 Cadmium 3 3 -- -- <1.0 <1.0 Chromium 4 3 -- -- 1.0 <5.0 Cobalt 3 3 -- -- <2.0 <3.0 Copper 1031615<2.030 Iron 21 9 57 12 0.1 320 Lead 3 1 -- -- 1.0 10 Lithium 3 0 8.8 10 5.0 11 Manganese 18 13 13 4.3 <0.02 120 Mercury 3 3 -- -- <0.1 <0.1 Molybdenum 3 1 -- -- 6.0 13 Nickel 1 1 -- -- <4.0 <4.0 Selenium 4 0 1.5 1.5 0.3 2.6 Silver 3 2 -- -- <1.0 2.0 Strontium 3 0 626 510 268 1,100 Vanadium 3 2 -- -- <6.0 43 Zinc 10196613.1360 Inyan Kara aquifer Aluminum 65 45 12 5.1 <10 140 Arsenic 90 28 1.1 0.8 <0.5 17 Barium 75 2 12 5.0 <2.0 126 Boron 9201258030670 Cadmium 13 12 -- -- <2.0 <2.0 Chromium 62 56 -- -- <4.0 5.0 Cobalt 62 38 2.0 1.2 <2.0 19 Copper 84 66 4.7 0.2 <2.0 210 Iron 145 20 460 33 1.2 3,600 Lead 1 0 -- -- 15 15 Lithium 78 0 103 79 19 455 Manganese 131 21 92 43 <2.0 1,424 Mercury 27 23 1-- 1-- <0.1 1.9 Molybdenum 78 47 5.2 4.0 <1.0 21 Nickel 62 52 1-- 1-- <4.0 9.0 Selenium 89 16 1.6 0.6 <0.2 23 Silver 62 53 1-- 1-- <2.0 4.0 Strontium 79 0 2,133 1,500 31 8,460 Vanadium 79 56 2.2 1.3 <1.0 19 Zinc 90 16 78 20 <3.0 1,803 78 Water-Quality Characteristics in the Black Hills Area, South Dakota Table 7.