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Assessing Groundwater Contamination Risks in California's Disadvantaged Communities

OWP at Sacramento State


Many communities in California rely on groundwater supplies that are contaminated with pollutants such as metals, nitrates, and hydrocarbons. For low-income and disadvantaged communities with limited access to water treatment technologies, this is an especially challenging problem that puts residents at risk.

To help identify communities at risk of groundwater contamination, the Office of Water Programs at Sacramento State University used openly available data for groundwater monitoring and social and economic indicators to map groundwater contamination risk across the state.

The interactive maps featured below are designed to help managers and policymakers identify disadvantaged communities (DACs) and severely disadvantaged communities (SDACs) that physically reside over contaminated groundwater basins and may use these basins for drinking water supplies. As specified in section 79505.5 of the California Water Code, such communities are defined as:

The interactive maps can help direct investments to clean-up contaminated groundwater wells using current and future funding. For instance, the Water Quality, Supply, and Infrastructure Act of 2014, also referred to as Proposition 1 (or Prop 1), authorized $7.545 billion in general bonds to California water projects ranging from surface and groundwater storage to watershed and drinking water protection. Prop 1 supports a multidisciplinary technical assistance program to help small, disadvantaged communities (< 10,000 people) in developing, funding, and implementing drinking water, wastewater, stormwater, and groundwater capital projects. Better identifying at-risk communities that need such funds was a motivator in creating this web-based application.

*According to the US Census Bureau's 2016 American Community Survey five-year estimate,California's MHI is $63,783.

The California Groundwater Contamination Risk Index (GRID)

The California Groundwater Contamination Risk Index (GRID) is a groundwater quality assessment tool that aggregates data to assess the severity of risk for contaminated groundwater in relation to social and economic indicators at high geographic resolution (U.S. Census block groups). GRID is a user-friendly screening tool designed to help water managers and policymakers identify communities with polluted groundwater that are eligible for Prop 1 technical assistance grants and similar funding sources intended for groundwater cleanup.

DATA

GRID was developed using water quality data analyzed from California's Groundwater Ambient Monitoring & Assessment (GAMA) Program. The GAMA data used here is based on sampling for ten key groundwater pollutants (e.g. arsenic, nitrate, hexavalent chromium) from 99,083 groundwater wells across the state in the past year. Table 1 lists the chemical pollutants considered in the index.

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METHOD

The single-factor index method (Prati et al., 1971; Yan et al., 2015; Zu-xin, 2005) was used to assess contaminant-specific water quality in census block groups. For all wells in a block group area, single-factor pollution indices (Pi) for each contaminant were generated for the ten contaminants using the formula:

Pi = Ci/MCLi

where,

Pi is the pollution index for a designated contaminant i
Ci is the measured concentration of contaminant i
MCLi is the maximum contaminant level of contaminant i

Smaller Pi values correspond to better water quality. A Pi value less than or equal to 1 indicates compliance with legal threshold limits for the allowable concentration of a contaminant in drinking water supplies. Alternatively, a Pi value greater than 1 indicates non-compliance with legal threshold limits, with the severity of groundwater contamination increasing with higher Pi values. Many block groups contained only one groundwater well with reportable data through GAMA, but for block groups with multiple wells, reported values were averaged across wells to create a single value in the block group to compare with the MCL.

To jointly assess the contamination risks for multiple pollutants, the contaminant-specific pollution indices were aggregated into a composite index (i.e. GRID) using simple summation:

GRID =Σ Ci / MCLi

where,

GRID is the California Groundwater Risk Index
n is the number of contaminants included in the composite index
Ci is the measured concentration of contaminant i
MCLi is the maximum contaminant level of contaminant i

A quantile classification scheme (Table 2) was used to rank GRID outputs. This composite index is shown on maps below, while data for the contaminant-specific indices can be found in the supplemental data files available for download.

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Mapping Groundwater Contamination Risk Using GRID

Map 1: Groundwater Contamination Throughout the State

The following map highlights the distribution and severity of groundwater contamination risk according to block groups across Califonia, independent of median household income and DAC status. Values are based on the composite GRID index that includes the ten pollutants from Table 1. Areas with no available groundwater quality data are not shaded.

Importantly, the GRID maps are a screening tool. For large block groups (sparsely populated areas), the entire groundwater basin is not necessarily at risk of contamination, but it is the smallest geographic unit that can be mapped to coordinate groundwater contamination risk and low-income communities. Further in-basin data would refine locations of interest.

Clicking on a selected block group displays its geographic identification, median household income, DAC status, and groundwater quality classification.



Map 2: DACs with High Groundwater Contamination Risk (Pollutant Index > 1.8)

The following map displays DACs with high and serious groundwater contamination risk, based on the composite GRID index that includes the ten pollutants from Table 1.

Clicking on a selected block group will display the respective block group's geographical identification, median household income, DAC status, and groundwater quality classification. 1. Areas with no available groundwater quality data are not shaded.




Map 3: SDACs with High Groundwater Contamination Risk (Pollutant Index > 1.8)

The following map displays SDACs with high and serious groundwater contamination risk, based on the composite GRID index that includes the ten pollutants from Table 1.

Clicking on a selected block group will display the respective block group's geographical identification, median household income, DAC status, and groundwater quality classification. 1. Areas with no available groundwater quality data are not shaded.




Map 4: MCL Exceedance by the Top 10 Contaminants in the State

The following map displays the number of contaminants (Table 1) that are close to or exceeding MCL thresholds for each Census block group.

Clicking on a selected block group will display the respective block group’s geographic and DAC status identification, the number of contaminants exceeding MCLs, and MCL exceedances by individual contaminants. Areas with no available groundwater quality data are not shaded.



Supplemental Data

1,2,3 Trichloropropane (1,2,3 TCP)
123TCP_All Data.rar
123TCP_DAC_HP.rar
123TCP_DAC_MP.rar
123TCP_DAC_SP.rar
123TCP_SDAC_HP.rar
123TCP_SDAC_MP.rar
123TCP_sDAC_SP.rar
1,2-Dibromo-3-chloropropane (DBCP)
DBCP_All_Data.rar
DBCP_DAC_HP.rar
DBCP_DAC_MP.rar
DBCP_DAC_SP.rar
DBCP_SDAC_HP.rar
DBCP_SDAC_MP.rar
DBCP_SDAC_SP.rar
Arsenic
Ars_All_Data.rar
Ars_DAC_HP.rar
Ars_DAC_MP.rar
Ars_DAC_SP.rar
Ars_SDAC_HP.rar
Ars_SDAC_MP.rar
Ars_SDAC_SP.rar
GRID_Final Index
GRID_AllData.rar
GRID_DAC_HP.rar
GRID_DAC_MP.rar
GRID_DAC_SP.rar
GRID_SDAC_HP.rar
GRID_SDAC_MP.rar
GRID_SDAC_SP.rar
Hexavalent Chromium (CR6)
Cr6_All_Data.rar
Cr6_DAC_HP.rar
Cr6_DAC_MP.rar
Cr6_DAC_SP.rar
Cr6_SDAC_HP.rar
Cr6_SDAC_MP.rar
Cr6_SDAC_SP.rar
Nitrate as N
N_All_Data.rar
N_DAC_HP.rar
N_DAC_MP.rar
N_DAC_SP.rar
N_SDAC_HP.rar
N_SDAC_MP.rar
N_SDAC_SP.rar
Perchlorate
PerChl_All_Data.rar
PerChl_DAC_HP.rar
PerChl_DAC_MP.rar
PerChl_DAC_SP.rar
PerChl_SDAC_HP.rar
PerChl_SDAC_MP.rar
PerChl_SDAC_SP.rar
Tetrachloroethene (PCE)
PCE_All_Data.rar
PCE_DAC_HP.rar
PCE_DAC_MP.rar
PCE_DAC_SP.rar
PCE_SDAC_HP.rar
PCE_SDAC_MP.rar
PCE_SDAC_SP.rar
Total Dissolved Solids (TDS)
TDS_All_Data.rar
TDS_DAC_HP.rar
TDS_DAC_MP.rar
TDS_DAC_SP.rar
TDS_SDAC_HP.rar
TDS_SDAC_MP.rar
TDS_SDAC_SP.rar
Trichloroethene (TCE)
TCE_Add_Data.rar
TCE_DAC_HP.rar
TCE_DAC_MP.rar
TCE_DAC_SP.rar
TCE_SDAC_HP.rar
TCE_SDAC_MP.rar
TCE_SDAC_SP.rar
Uranium
Ur_All_Data.rar
Ur_DAC_HP.rar
Ur_DAC_MP.rar
Ur_DAC_SP.rar
Ur_SDAC_HP.rar
Ur_SDAC_MP.rar
Ur_SDAC_SP.rar

References

About OWP at Sacramento State

The Office of Water Programs (OWP) at California State University, Sacramento (Sacramento State) provides training, technical assistance, and applied research management services for a variety of water resource and water quality disciplines: drinking water, wastewater, stormwater, watershed planning, and soils and groundwater. The mission of OWP is to provide cost-effective solutions for protecting and enhancing water resources, public health, and the environment.

Since 2016, OWP has served as the Environmental Protection Agency's (EPA) Region 9 Environmental Finance Center (EFC). The EFC assists the region’s rural, disadvantaged, and tribal communities in financial planning and asset management, supporting development of internal capacity to sustainably fund environmental and public health services. We focus on providing technical assistance to communities for water management using our expertise in applied research and program administration. We work with state government agencies and local water utilities throughout California and the western United States.


c.2018-19 OWP at Sacramento State