Environmental justice and drinking water quality: are there socioeconomic disparities in nitrate levels in U.S. drinking water?

Background Low-income and minority communities often face disproportionately high pollutant exposures. The lead crisis in Flint, Michigan, has sparked concern about broader socioeconomic disparities in exposures to drinking water contaminants. Nitrate is commonly found in drinking water, especially in agricultural regions, and epidemiological evidence suggests elevated risk of cancer and birth defects at levels below U.S. EPA’s drinking water standard (10 mg/L NO3-N). However, there have been no nationwide assessments of socioeconomic disparities in exposures to nitrate or other contaminants in U.S. drinking water. The goals of this study are to identify determinants of nitrate concentrations in U.S. community water systems (CWSs) and to evaluate disparities related to wealth or race/ethnicity. Methods We compiled nitrate data from 39,466 U.S. CWSs for 2010–2014. We used EPA’s Safe Drinking Water Information System (SDWIS) to compile CWS characteristics and linked this information with both city- and county-level demographic data gathered from the U.S. Census Bureau. After applying multiple imputation methods to address censored nitrate concentration data, we conducted mixed-effects multivariable regression analyses at national and regional scales. Results 5.6 million Americans are served by a CWS that had an average nitrate concentration ≥ 5 mg/L NO3-N between 2010 and 2014. Extent of agricultural land use and reliance on groundwater sources were significantly associated with nitrate. The percent of Hispanic residents served by each system was significantly associated with nitrate even after accounting for county-level cropland and livestock production, and CWSs in the top quartile of percent Hispanic residents exceeded 5 mg/L nearly three times as often as CWSs serving the lowest quartile. By contrast, the percent of residents living in poverty and percent African American residents were both inversely associated with nitrate. Conclusions Epidemiological evidence for health effects associated with drinking water above 5 mg/L NO3-N raises concerns about increased risk for the 5.6 million Americans served by public water supplies with average nitrate concentrations above this level. The associations we observed between nitrate concentrations and proportions of Hispanic residents support the need for improved efforts to assist vulnerable communities in addressing contamination and protecting source waters. Future studies can extend our methods to evaluate disparities in exposures to other contaminants and links to health effects. Electronic supplementary material The online version of this article (10.1186/s12940-018-0442-6) contains supplementary material, which is available to authorized users.

WS.PWS_DEACTIVATION_DATE --The date in which the water system was reported as being closed/deactivated. PWS_TYPE_CODE --A system--generated coded value which classifies the water system according to federal requirements. It includes Community Water Systems (CWS), Non--Transient Non--Community Water Systems (NTNCWS), and Transient Non--Community Water Systems (TNCWS).
WS.GW_SW_CODE --Indicates if the water system is considered having ground water (GW) or surface water (GW) source under SDWA.

WS.PRIMARY_SOURCE_CODE --
The code showing the differentiation between the sources of water: ground water (GW), groundwater purchased (GWP), surface water (SW), surface water purchased (SWP), groundwater under influence of surface water (GU), or purchased ground water under influence of surface water source (GUP).
WS.POPULATION_SERVED_COUNT --Water system's estimate of the number of people served by the system.
WS.SERVICE_CONNECTIONS_COUNT --Number of service connections to the water system. AREA_TYPE_CODE --Study created variable categorizing the available documentation for the type area served by a water system.
• CN --System serves a single county • CN & ZC --System serves a single county and a single zip code • CT --System serves a single city • CT & CN --System serves a single city and a single county • multiCN --System serves multiple counties (and up to one city and/or zip code) • multiCT --System serves multiple cities (and up to one county and/or zip code) • multimulti --System serves two or more regions of at least two area types (city, county, zip code) • multiZC --System serves multiple zip codes (and up to one city and/or county) • No Geo Available --No information provided on the area served by the water system in either the Water System or Geographic Area modules CITY_SERVED --The city served by a water system. Multiple cities are separated by a comma (",").
COUNTY_SERVED --The county served by a water system. Multiple counties are separated by a comma (",").
ZIP_SERVED --The zip code served by a water system. Multiple zip codes are separated by a comma (","). WS.IS_GRANT_ELIGIBLE_IND --Code that indicates if the primacy agency has reported the minimum necessary data elements for this water system to include it in grant calculations.

WS.IS_SCHOOL_OR_DAYCARE_IND --
Code that indicates if the water system's primary service area is a school or daycare as defined by EPA/OGWDW.
SYS_ONLY --Study created variable. Was the system found in the Water System module but not the Geographic Area module (Y/N)?
GEO_ONLY --Study created variable. Was the system found in the Geographic Area module but not the Water System module (Y/N)?

CWS facilities & assoc. sellers
This file includes information provided from the SDWIS API's (basic documentation found here: https://www.epa.gov/enviro/sdwis--model) Water System Facility module, which details various characteristics of each U.S. water system facility including information on which water systems sell water to facilities. All facilities in this database were active during the 2010--2014 study period. This includes all presently active facilities as well as facilities that were deactivated during or after 2010. This database has been subset to those CWS facilities that purchase water from selling system that is among the 42,114 CWS that were active during our study period (2010--2014) and do not purchase their water (i.e. are not indicated as GWP, GUP, or SWP in the WS.PRIMARY_SOURCE_CODE field). In our study, we only associated purchasing system demographics with the wholesaler when the two systems were connected via a permanent service connection (AVAILABILITY_CODE = "P") as all other connections were less consistent and it could not be determined if the wholesaler reliably provided water to the purchasing community year--round.

Variable definitions:
PWSID --The national identification number for the Public Water System which uniquely identifies the water system within a specific state. Format: SSXXXXXXXXXX where: SS = the Federal Information Processing Standard (FIPS) Pub 5--2 State abbreviation in which the water system is located, or the region number of the EPA region responsible for an Indian reservation, and XXXXXXXXXX = the water system identification code assigned by the State. In this database, represents the buying facility's PWSID.
PRIMACY_AGENCY_CODE --Two character postal code for the state or territory having regulatory oversight for the water system. In this dataset, will only represent a state, but in other datasets can represent the two--digit EPA Region number (if the system is regulated directly by EPA) or NN for Navajo Nation.
PWS_ACTIVITY_CODE --Code that indicates the current activity status of the public water system.
• A --Active • I/N --Inactive PWS_DEACTIVATION_DATE --The date in which the water system was reported as being closed/deactivated.

PWS_TYPE_CODE --A system--generated coded value which classifies the water system according to federal requirements. It includes Community Water Systems (CWS), Non--Transient Non--Community Water Systems (NTNCWS), and Transient Non--Community Water Systems (TNCWS).
FACILITY_ID --Water system facility ID that, when used with the PWSID, uniquely identifies a water system facility.
FACILITY_NAME --Name of water system facility.
FACILITY_TYPE_CODE --Code that Indicates the type of water system facility.
FACILITY_ACTIVITY_CODE --Code that indicates the current activity status of the facility. SELLER_PWSID --PWSID of the water system that is selling water to this system through this interconnection. Is also the "upstream" water system to the parent of this facility. SELLER_PWS_NAME --PWS Name of the water system that is selling water to this system through this interconnection. Is also the "upstream" water system to the parent of this facility.
SELLER_TREATMENT_CODE --Code that indicates whether the seller is or is not treating the source or whether the seller treatment status is unknown. Applies only to source facilities.
• F --Treated by seller including SWT • G --Treated by seller with 4--log virus for GWR • N --No, not treated • Y --Partially treated by seller

Service areas of purchasing CWSs
This file includes information provided from the SDWIS API's (basic documentation found here: https://www.epa.gov/enviro/sdwis--model) Service Area module, which details type of areas served by water systems in the U.S. Included in this dataset are all "primary service areas" (or all service areas in circumstances where no service area was designated as a primary) for the purchasing systems connected to a wholesaler through a permanent service connection. In our analysis, we only associated purchasing system demographics with the wholesaler when the purchasing system served a "Residential Area" or "Municipality" (in the SERVICE_AREA_DESC field), as other areas were not thought to be representative of the entirety of the demographics of the purchasing community's cities served.
Variable Definitions: PWSID --The national identification number for the Public Water System which uniquely identifies the water system within a specific state. Format: SSXXXXXXXXXX where: SS = the Federal Information Processing Standard (FIPS) Pub 5--2 State abbreviation in which the water system is located, or the region number of the EPA region responsible for an Indian reservation, and XXXXXXXXXX = the water system identification code assigned by the State. In this database, represents the buying facility's PWSID.
PRIMACY_AGENCY_CODE --Two character postal code for the state or territory having regulatory oversight for the water system. In this dataset, will only represent a state, but in other datasets can represent the two--digit EPA Region number (if the system is regulated directly by EPA) or NN for Navajo Nation.
PWS_ACTIVITY_CODE --Code that indicates the current activity status of the public water system.
• A --Active • I --Inactive PWS_TYPE_CODE --A system--generated coded value which classifies the water system according to federal requirements. It includes Community Water Systems (CWS), Non--Transient Non--Community Water Systems (NTNCWS), and Transient Non--Community Water Systems (TNCWS).
SERVICE_AREA_DESC --Description of service area.
IS_PRIMARY_SERVICE_AREA_CODE --Indicated "Y" if the service area in question is the primary area type served by the system. Otherwise, all service areas associated with a system are listed.