Laserfiche WebLink
Water Quality 15 <br />Anthropogenic sources of nitrogen and phosphorus include <br />synthetic fertilizers, animal waste, and septic-system effluent. <br />Nitrogen also is a by-product of the combustion of fossil fuels. <br />Nitrogen and phosphorus are essential nutrients for plant <br />growth, but high concentrations of either nutrient in a water <br />body can cause unwanted, dense algal blooms. High concentra- <br />tions of nitrate in drinking water can cause methemoglobinemia <br />(blue-baby syndrome) in small children (Hem, 1992). Drinking- <br />water standards or health advisories have been established for <br />ammonia (un-ionized ammonia [NH31 plus ammonium <br />[NH4+1), nitrite, and nitrate (table 2). <br />Ground-water samples were analyzed for the dissolved <br />forms of ammonia (un-ionized ammonia plus ammonium), <br />ammonia plus organic nitrogen, nitrite, nitrate, phosphorus, and <br />orthophosphate. <br />Ammonia (un-ionized ammonia plus ammonium) was <br />detected in low concentrations in water samples from the <br />11 wells (table 2). Most (60 of 70) ammonia concentrations <br />were less than 0.01 mg/L (table 5, Appendix I). Ammonia also <br />was detected in 32 percent (7 of 22) of the blank samples, pri- <br />marily at low concentrations (0.002-0.005 mg/L); one sample <br />had a concentration of 0.021 mg/L (table 9, Appendix II). The <br />presence of ammonia in some blanks indicates that small <br />amounts of ammonia were introduced during sampling and(or) <br />laboratory analysis in a few samples. Also, variability in <br />reported ammonia concentrations was within the concentration <br />range of the environmental data, and there was inconsistency in <br />the detection of ammonia in replicate-pair samples (table 7, <br />Appendix II). Because of these factors, the exact concentration <br />of ammonia in the ground-water samples collected during the <br />sampling period is uncertain. It is known, however, that only <br />small amounts of ammonia were present in the samples, and all <br />concentrations were less than or equal to about 0.040 mg/L <br />(table 2). <br />In surface or ground water, nitrite usually is undetected or <br />detected at very low concentrations because it is rapidly con- <br />verted to nitrate in the presence of oxygen. During this study, <br />nitrite concentrations were undetected or were less than or equal <br />to the reporting level of 0.001 mg/L in all ground-water samples <br />but one (0.004 mg/L for site 8 on 09/13/2000). Detected con- <br />centrations of nitrite were much lower than the USEPA MCL <br />for nitrite of 1 mg/L in drinking water (table 2). <br />Nitrate plus nitrite concentrations in the ground-water <br />samples were essentially nitrate, and will be considered as such <br />in this report, because of the very low or undetected concentra- <br />tions of nitrite. Nitrate concentrations in the ground-water sam- <br />ples ranged from less than 0.005 to 4.72 mg/L, with a median <br />concentration of 0.208 mg/L (table 2). Most (54 of 70) nitrate <br />concentrations were very low, less than 0.3 mg/L. Nitrate <br />concentrations for all water samples from site 7, completed in <br />the alluvial aquifer in Tabernash, were less than or equal to <br />0.008 mg/L. Nitrate in this well probably was denitrified to <br />reduced species of nitrogen; reducing conditions for this well <br />were indicated by low dissolved-oxygen concentrations and <br />elevated iron and manganese concentrations. Nitrate concentra- <br />tions were elevated (greater than 1 mg/L) in water samples from <br />three alluvial aquifer wells (sites 1, 4, and 8). Nitrate concentra- <br />tions greater than 2 mg/L may indicate nitrate introduced by <br />human activities (U.S. Geological Survey, 1999). Nitrate con- <br />centrations for all seven water samples from site 8 were above <br />2 mg/L, the estimated national background concentration of <br />nitrate in ground water (Mueller and Helsel, 1996). These con- <br />centrations may indicate that recent land-use practices have <br />affected water quality, as water in site 8 has an estimated chlo- <br />rofluorocarbon recharge date of the 1990's (table 1). Results of <br />the Wilcoxon rank-sum test indicated that the hypotheses of <br />similarities in nitrate concentrations between urban and non- <br />urban areas and between ISDS and non-ISDS areas could not be <br />rejected (table 3). Similarities in nitrate concentrations between <br />aquifer types also could not be rejected statistically (table 3) <br />because of comparable median values, although nitrate concen- <br />trations were elevated in water samples from some alluvial <br />aquifer wells (fig. 5). No nitrate concentrations were greater <br />than the USEPA MCL for nitrate in drinking water of 10 mg/L <br />(table II). Variability and uncertainty in nitrate concentrations <br />(table 8, Appendix II) had little, if any effect, on the nitrate <br />results. <br />Orthophosphate was the primary component of dissolved <br />phosphorus in water from the 11 wells in the study area, as indi- <br />cated by the similarities in concentrations of the two constitu- <br />ents. For some ground-water samples, the dissolved phosphorus <br />concentration was slightly less than the orthophosphate concen- <br />tration. Small differences in these two constituents may be a <br />reflection of analytical precision or variability in phosphorus <br />concentrations, as discussed in "Quality-Control Methods and <br />Analysis" in Appendix 11. Orthophosphate concentrations in the <br />ground-water samples ranged from an estimated value of about <br />0.005 to 0.137 mg/L, and the median concentration was <br />0.047 mg/L (table 2). Minimum concentrations were detected in <br />water samples from three alluvial aquifer wells (sites 1, 4, and <br />7). All samples for sites 2 and 10, completed in the Trouble- <br />some Formation aquifer, had orthophosphate concentrations <br />greater than 0.1 mg/L. Orthophosphate concentrations were <br />higher in water samples from the Troublesome Formation aqui- <br />fer and non-ISDS areas than in samples from the alluvial aquifer <br />and ISDS areas, as indicated by median concentrations <br />(table 3). The most common geologic source of phosphorus is <br />NUMBER OF SAMPLES <br />(39) (24) <br />w <br />F- <br />a U) <br />Q(D <br />Zccaw <br />02Fw-O <br />wJ J= <br />J_ <br />w Z <br />Q U) Za <br />U) - <br />0 <br />Alluvial Troublesome <br />Formation <br />AQUIFER TYPE <br />EXPLANATION <br />)K Outlier <br />90th percentile <br />75th percentile <br />50th percentile (median) <br />25th percentile <br />10th percentile <br />Figure 5. Distribution of dissolved nitrate concentrations by <br />aquifer type for wells in the Fraser River watershed, 1998-2001.