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:
- DACs are communities with a median household income (MHI) less than 80% of the statewide average*
(MHI<$51,026).
- SDACs are communities with an MHI less than 60% of the statewide average* (MHI<$38,270).
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.
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.
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 California, 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
References
- Prati, L., Pavanello, R., & Pesarin, F. (1971). Assessment of surface water quality by a single index
of pollution. Water research, 5(9), 741-751.
- U.S. Geological Survey, 2018, California Groundwater Ambient Monitoring and Assessment (GAMA) Program
Priority Basin Project-Shallow aquifer assessment (ver. 1.1, September 2018): U.S. Geological Survey
Fact Sheet 2012-3136, 2 p.
- United States Census Bureau / American FactFinder.B11001 : Household Type (Including Living Alone).
2007-2011 American Community Survey. U.S. Census Bureau American Community Survey Office, 2011. Web. 1
January 2013.
- Yan, C.-A., Zhang, W., Zhang, Z., Liu, Y., Deng, C., & Nie, N. (2015). Assessment of water quality and
identification of polluted risky regions based on field observations & GIS in the honghe river
watershed, China. PloS one, 10(3), e0119130.
- Zu-xin, X. (2005). Comprehensive Water Quality Identification Index for Environmental Quality
Assessment of Surface Water. Tongji Daxue Xuebao/Journal of Tongji Universit. China. 33.4 (2005):
482-488.
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
