Page 78 - Beaumont Basin Watermaster

Section 4
Water Quality Conditions

objectives, and 5 exceeded the maximum benefit objective; two 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
2009-2013 reporting period for 44 wells including 21 potable wells. The maximum Nitrate-N
concentrations for production wells ranged from 1.1 to 8.7 mg/L and averaged 3.9 mg/L. Of the
22 production wells, four had a maximum concentration below the antidegradation objective,
11 wells were between the antidegradation and maximum benefit objectives, and six wells
exceeded the maximum benefit objective for the BMZ. Four of these wells (Sharondale Mesa
No. 2, BCVWD No. 16 and 21, and Rancho Calimesa Mobile Home Park No. 2) 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 23 non-potable wells ranged from 0.3 to 21.3 mg/L.
Average maximum concentrations for these wells (5.5 mg/L) exceeded the maximum benefit
objective for the BMZ. Of these wells, three had maximum concentrations below the
antidegradation objective, but nine exceeded the maximum benefit objective. In addition, five
of these wells had maximum concentrations equals to or higher than the primary federal and
state drinking water standard.

The highest Nitrate-N concentration within the Beaumont Basin was observed at BCVWD Well
No. 21 at 8.7 mg/L. Wells exceeding the current MCL are mainly located 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
California, Riverside and funded as a Supplemental Environmental Project by the State Water

Beaumont Basin Watermaster 2013 Annual Report – DRAFT August 2014 4-2