Needs_Assessment_Hexavalent_Chromium (MapServer)

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Service Description: By section, the water quality grade for hexavalent chromium, as determined by the methodology published in the Needs Analysis White Paper.

The State Water Resources Control Board (State Water Board) was tasked to conduct a Needs Assessment of the state of drinking water in California (SB 862, 2018). The State Water Board Division of Drinking Water (DDW) identified three elements for this analysis: (1) identification of public water systems at risk, (2) identification of domestic wells and state small systems at risk, and (3) an analysis of the cost to implement Human Right to Water. The information provided in this application supports the second element, identifying the location and number of domestic wells potentially accessing groundwater affected by constituents at concentrations above regulatory levels.

Since the water supply accessed by domestic wells is not regulated by the state, accurate locations and groundwater quality data is generally not available. The values presented in this application represent estimates of domestic well location density and groundwater quality. The information provided in this application should not be interpreted to represent measured data. Rather, this analysis is intended to help prioritize future sampling and characterization efforts and to support the cost analysis associated with the Needs Assessment.

Domestic depth groundwater quality estimates for six constituents are shown on the map: nitrate, arsenic, hexavalent chromium*, uranium, 1,2,3 trichloropropane (123 TCP), and perchlorate. For each constituent, groundwater quality estimates can be displayed for all Public Land Survey System (PLSS) sections statewide, or for only the PLSS sections that include at least one domestic well (domestic well counts and locations were obtained from the Department of Water Resources Online System of Well Completion Reports). Data fields for each section include the estimated average constituent concentration, the methodology used to calculate that average, the number of recent MCL/SMCL exceedances, and an overall water quality grade. The grade is a rated representation of the combined section detection and number of recent MCL/SMCL exceedances (see below). Section detections are represented by an MCL index, which is the constituent concentration divided by its regulatory threshold (MCL, SMCL, etc.). An MCL index of 1 reflects a value of the MCL/SMCL, while a 0.8 index represents a value of 80% the MCL/SMCL. The method indicates which of the three sources of data were used to estimate the section concentration: data from within the section, data from neighboring sections, or data from the groundwater unit. Other fields include area, the domestic well count, and the MTRS (the PLSS section number listed as meridian, township, range, and section).

Water Quality Grades:

6: Recent MCL exceedances > 0, average section detection > MCL

5: Recent MCL exceedances = 0, average section detection > MCL

4: Recent MCL exceedances > 0, average section detection < MCL

3: Recent MCL exceedances = 0, average section detection 80 – 100% of MCL

2: Recent MCL exceedances = 0, average section detection between 50 – 80% of MCL

1: Recent MCL exceedances = 0, average section detection < 50% of MCL

0: unknown water quality (no data available)

Ambient groundwater quality data from water supply well sources in the GAMA Groundwater Information System were processed through time and depth filters developed for this analysis in order to capture the depths accessed by domestic wells (by groundwater unit). This process allowed the analysis to include an increased amount of water quality data, and from sources typically not utilized for analyzing the domestic well water resource, creating a more robust analysis. Censored data (non-detect, below reporting, and zero-values) underwent a substitution process.

*For hexavalent chromium, a comparison value of 20 µg/L was used in place of an MCL.

Map Name: Hexavalent Chromium

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Copyright Text: For any questions or comments, please email Dori Bellan and/or Emily Houlihan, GAMA Program geologists. dorian.bellan@waterbpoards.ca.gov emily.houlihan@waterboards.ca.gov

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