Abstract

Onsite wastewater disposal systems (OWDS) are used extensively in the Black Hills of South Dakota where many of the watersheds and aquifers are characterized by fractured or solution-enhanced bedrock with thin soil cover. A study was conducted during 2006–08 to characterize water-quality effects and indicators of OWDS. Water samples were collected and analyzed for potential indicators of OWDS, including chloride, bromide, boron, nitrite plus nitrate (NO₂+NO₃), ammonia, major ions, nutrients, selected trace elements, isotopes of nitrate, microbiological indicators, and organic wastewater compounds (OWCs). The microbiological indicators were fecal coliforms, Escherichia coli (E. coli), enterococci, Clostridium perfringens (C. perfringens), and coliphages. Sixty ground-water sampling sites were located either downgradient from areas of dense OWDS or in background areas and included 25 monitoring wells, 34 private wells, and 1 spring. Nine surface-water sampling sites were located on selected streams and tributaries either downstream or upstream from residential development within the Precambrian setting. Sampling results were grouped by their hydrogeologic setting: alluvial, Spearfish, Minnekahta, and Precambrian.

Mean downgradient dissolved NO₂+NO₃ concentrations in ground water for the alluvial, Spearfish, Minnekahta, and Precambrian settings were 0.734, 7.90, 8.62, and 2.25 milligrams per liter (mg/L), respectively. Mean downgradient dissolved chloride concentrations in ground water for these settings were 324, 89.6, 498, and 33.2 mg/L, respectively. Mean downgradient dissolved boron concentrations in ground water for these settings were 736, 53, 64, and 43 micrograms per liter (µg/L), respectively. Mean dissolved surface-water concentrations for NO₂+NO₃, chloride, and boron for downstream sites were 0.222 mg/L, 32.1 mg/L, and 28 µg/L, respectively.

Mean values of δ¹⁵N and δ¹⁸O (isotope ratios of ¹⁴N to ¹⁵N and ¹⁸O to ¹⁶O relative to standard ratios) for nitrate in ground-water samples were 10.4 and -2.0 per mil (‰), respectively, indicating a relatively small contribution from synthetic fertilizer and probably a substantial contribution from OWDS. The surface-water sample with the highest dissolved NO₂+NO₃ concentration of 1.6 mg/L had a δ¹⁵N value of 12.36 ‰, which indicates warm-blooded animals (including humans) as the nitrate source.

Fecal coliforms were detected in downgradient ground water most frequently in the Spearfish (19 percent) and Minnekahta (9.7 percent) settings. E. coli was detected most frequently in the Minnekahta (29 percent) and Spearfish (13 percent) settings. Enterococci were detected more frequently than other microbiological indicators in all four settings. Fecal coliforms and E. coli were detected in 73 percent and 95 percent of all surface-water samples, respectively. Enterococci, coliphages (somatic), and C. perfringens were detected in 50, 70, and 50 percent of surface-water samples, respectively.

Of the 62 OWC analytes, 12 were detected only in environmental samples, 10 were detected in at least one environmental and one blank sample (not necessarily companion pairs), 2 were detected only in blank samples, and 38 were not detected in any blank, environmental, or replicate sample from either ground or surface water. Eleven different organic compounds were detected in ground-water samples at eight different sites. The most frequently occurring compound was DEET, which was found in 32 percent of the environmental samples, followed by tetrachloroethene, which was detected in 20 percent of the samples. For surface-water samples, 16 organic compounds were detected in 9 of the 10 total samples. The compound with the highest occurrence in surface-water samples was camphor, which was detected in 50 percent of samples.

The alluvial setting was characterized by relatively low dissolved NO₂+NO₃ concentrations, detection of ammonia nitrogen, and relatively high concentrations of major ions, particularly sulfate and sodium, compared to other settings. Nitrogen and oxygen isotope results indicated that denitrification was occurring in water from a few wells. Chloride concentrations at downgradient sites were substantially higher than background concentrations.

The Spearfish setting was characterized by consistently high dissolved NO₂+NO₃ concentrations—75 percent of samples had concentrations higher than 5 mg/L—and relatively frequent detections of microbiological indicators and OWCs. Nitrogen isotope results indicated that the source of nitrogen from two of the five sites probably included some nitrogen from synthetic sources mixed with nitrogen from warm-blooded animals. Although concentrations of chloride, boron, and bromide generally were higher than background concentrations, the variability in the concentrations in proximal areas complicated interpretations from these OWDS indicators.

In the Minnekahta setting, most indicator concentrations in downgradient samples were higher than background concentrations, and correlations between indicators in the same sample were strong. Mean, median, and maximum dissolved NO₂+NO₃ concentrations were 8.62, 7.42, and 24.3 mg/L, respectively, for the Minnekahta setting—the highest of the four hydrogeologic settings. Nitrogen isotope results indicated that the probable source was warm-blooded animals (predominantly from OWDS effluents) in 17 of 23 samples. OWCs were detected in more than one-half of the samples, and E. coli and coliphages were detected more frequently in the Minnekahta setting than in any other hydrogeologic setting.

Effects of OWDS varied among the different Precambrian study areas, but water-quality evidence indicated that water samples from the Precambrian wells generally were not as affected by anthropogenic influences as water samples from other hydrogeologic settings. However, the direct comparison of OWDS indicator results to other settings is questionable, as all of the wells in the Precambrian setting were previously established private drinking-water sources in contrast to the strategically placed monitoring wells used in the other three settings. Maximum dissolved NO₂+NO₃ concentrations for three sites in study area 4 that ranged from 5.06 to 8.11 mg/L probably included OWDS effects in the Precambrian setting. Microbiological indicators detected include fecal coliforms and E. coli, each detected in 2.6 percent of ground-water samples. No OWCs were detected in any of the six samples. Differences in chemical indicators of OWDS effects between upstream and downstream surface-water sites were not always evident; however, statistical tests on dissolved chloride, dissolved boron, fecal coliforms, and E. coli data indicated that results for upstream sites were significantly different than those for downstream sites.

Densities of OWDS were calculated for nine estimated contributing areas to the ground water near selected sites in the Spearfish, Minnekahta, and Precambrian settings. A positive correlation between downgradient dissolved NO₂+NO₃ concentration and OWDS density was observed. The analysis indicated a 90-percent chance that dissolved NO₂+NO₃ concentrations would be between 3 and 8 mg/L in an area with an OWDS density of 150 per square mile.

On the basis of background concentrations, dissolved NO₂+NO₃ concentrations higher than 2 mg/L were assumed to potentially indicate anthropogenic influence. Concentrations were higher than 2 mg/L in 75 percent of the ground-water samples, higher than 5 mg/L in 50 percent, and higher than 10 mg/L in 18 percent. Dissolved chloride concentrations, when compared to a mean background concentration of 14.6 mg/L, were excellent indicators of anthropogenic influence. Surface-water indicator results may give more insight into the effects of residential activities in general, as direct runoff probably contributes a greater proportion of the indicator load than do OWDS effluents.

First posted December 24, 2008