Nitrate, Arsenic and Selenium Concentrations in The Pecos Valley Aquifer, West Texas, USA Page: 232
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Nitrate, Arsenic and Selenium in Pecos Valley Aquifer
Institute, Redlands, California) was used to map
well locations and solute concentration categories.
RESULTS & DISCUSSION
Typical of water-quality data, solute concen-
trations were non-normally distributed; median con-
centrations were closer to minimum than maxi-
mum values (Table 1). Distribution of well depths
showed a similar pattern. Based upon observed,
non-normal solute-concentration and well-depth
distributions, non-parametric tests (Spearman and
Kruskal-Wallis) were used to compare and evalu-
ate associations between study variables.
Total dissolved solids (TDS) concentrations
varied widely, ranging from low concentrations
typically found in groundwater to extremely high
concentrations (Table 1). Most TDS observations
were high relative to typical (potable) groundwa-
ter; the median value was 2,687 mg/L (Table 1).
Approximately 92% of observations exceeded the
secondary drinking water standard of 500 mg/L,
and 74% of observations exceeded 1,000 mg/L,
or double the standard for TDS. Most observa-
tions exceeded 1,920 mg/L TDS - concentrations
above this amount frequently cause severe prob-
lems for crops (Bouwer, 1978).Table 1. Summary of Well Depth and Solute
Concentrations
N Minimum Median Maximum
Depth (m) 72 6 75 354
TDS 62 216 2,687 10,508
(mg/L)
Nitrate 62 <0.44 8.97 266
(mg/L)
Arsenic 79 <1 1 26
( g/L)
Selenium 79 <1 4 117
(pg/L)
TDS concentrations were high in several parts of
the study area, with multiple clusters near the
Pecos River (Fig. 2). One well, in the northwest-
ern corner of the study area, registered an ex-
tremely high TDS of 10,508 mg/L. Along the
Pecos River and in other irrigated parts of the
study area, high evapo-transpiration rates create
potential for salt accumulation in the soil. Subse-
quently, these salts may leach to groundwater with
irrigation return flow. Potentially, oilfield brine is
an additional source of dissolved solids in the study
area. Both past and presently producing oil wells
are scattered over the study area.N
Fig. 2. Total dissolved solids (TDS) concentrations; from smallest to largest, circles indicate 216-500, 501-
1,000, 1,001-5,000, and 5,001-10,500 mg/L232
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Hudak, Paul F. Nitrate, Arsenic and Selenium Concentrations in The Pecos Valley Aquifer, West Texas, USA, article, November 12, 2009; [Tehran, Iran]. (https://digital.library.unt.edu/ark:/67531/metadc152432/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.