Page 73 - Beaumont Basin Watermaster - 2015 Annual Report

2008-11 Engineering Report. Of the 26 non-domestic wells, 3 had a maximum TDS
concentration below the anti-degradation objective, 13 wells were between the anti-
degradation and maximum benefit objectives, and 13 exceeded the maximum benefit
objective; nine of these wells also exceeded the secondary drinking standard of 500 mg/L.

Most of the wells with the highest TDS concentrations, include those that exceeded drinking
water standards are located within the BMZ, but outside the Beaumont Basin.

4.1.2 Nitrate-Nitrogen

Figure 4-3 shows the maximum Nitrate-N concentrations measured at Basin wells during the
2011-2015 reporting period for 62 wells including 30 domestic wells owned by Appropriators or
by Overlying Parties. The maximum Nitrate-N concentrations for domestic wells ranged from
1.04 to 8.65 mg/L and averaged 2.83 mg/L. Of these wells, seven had a maximum
concentration below the anti-degradation objective of 1.5 mg/L, 20 wells had a maximum
concentration between the anti-degradation and maximum benefit objective of 5.0 mg/L, and
ten wells exceeded the maximum benefit objective for the BMZ. Only one of these wells
(BCVWD 21) also exceeded the 80 percent MCL for Nitrate-N, which is considered as a
threshold level that CDPH uses to begin considering potential blending and/or treatment
alternatives to address high nitrate concentrations in drinking water. None of the potable wells
exceeded the primary federal and state drinking water standard for nitrate-nitrogen of 10 mg/L.

Maximum Nitrate-N concentrations in the 32 non-domestic wells ranged from 0.49 to 47 mg/L.
Average maximum concentrations for these wells (8.47 mg/L) exceeded the maximum benefit
objective for the BMZ. Of these wells, five had maximum concentrations below the anti-
degradation objective, nine had maximum concentrations between the anti-degradation and
the maximum benefit objective, and the remaining 18 above the maximum benefit objective.
In addition, 11 of these wells had maximum concentrations equal to or higher than the primary
federal and state drinking water standard.

The highest Nitrate-N concentration within the Beaumont Basin was observed at a private well
with a concentration of 47 mg/L; this well is located a little more than half a mile north of
BCVWD Well No. 29 in the Singleton Basin. Other wells exceeding the current MCL are
mainly located south of the basin in the South Beaumont Basin.

4.1.3 Nitrate Studies in the Beaumont Management Zone
Rising nitrate concentrations observed in 2005 along the northern portion of the Basin
prompted STWMA to launch an investigation in 2006 to determine the potential impact on
groundwater quality from on-site waste disposal systems (OSWDS) commonly used in the
Cherry Valley Community of Interest (CVCOI). STWMA retained the services of Wildermuth
Environmental Inc. (WEI) to conduct this study.

The results of this study were disputed by the Beaumont Board of Supervisors’ Groundwater
Quality Evaluation Committee (Committee) as they identified potential shortcomings in
sampling design and project execution. The Committee recommended that an independent
assessment be conducted. They recommended that the second study should expand the
study area, consider reasonable build-out projections and other sources of groundwater
contamination. This independent study was conducted by scientist at the University of

Beaumont Basin Watermaster 2015 Annual Report – Final – December 2016 4-2