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.
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