Agricultural Land Use and Domestic Well Contamination in California

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Research Question

How is groundwater quality for domestic users affected by proximity to, and scale of, agricultural and ranching operations in California?

Data Sources

Name	Source	Year	Link	Data Format	Attributes/Fields	Description
GAMA Groundwater Info	USGS Priority Basin Project	2023	GAMA Groundwater	Point Vector	GM_WELL_ID, GM_CHEMICAL_NAME, GM_RESULT	Measures 199 contaminants from wells representing 95% of CA groundwater.
15_Crop_Mapping_2023	Land IQ, CA Dept. of Water Resources	2024	Land Use Mapping	Polygon Vector	CROP_TYPE, Shape_Area, MAIN_CROP	446,000+ field-classified polygons by crop type.
BLM Range Allotments	Bureau of Land Management	2022–24	BLM Range	Polygon Vector	Shape_Area	Grazing allotments defined spatially in CA.
Groundwater Basins (Bulletin 118)	CA Dept. of Water Resources	2021–22	i08 B118	Polygon Vector	FID_Groundwater_basins, Basin_area_Km^2	Official basin boundaries across CA.
Pesticide Use Data	CA Dept. of Pesticide Regulation	2022	Pesticide Summary	Polygon Vector	POTASSIUM NITRATE, AMMONIUM NITRATE, etc.	Acres treated & application rates per chemical and county.

Methods Overview

Our analysis used a series of spatial operations in ArcGIS ModelBuilder and Geoprocessing tools. Key components included land classification, contamination filtering, proximity analysis, and regression modeling.

Part 1: Agricultural Land Area Calculation

We used Select, Intersect, Dissolve, and Field Calculator to calculate the percent area of cropland and ranchland per groundwater basin.

Part 2: Domestic Wells and Land Use

Using Spatial Join, Union, and Clip, we filtered and merged domestic wells with cropland and ranchland overlays to assess shared basin locations.

Part 3: Nitrate and Other Contaminants

Wells were filtered for key contaminants. Nitrate emerged as most significant (EPA MCL >10 mg/L), and wells exceeding this were isolated for further analysis.

Part 4: Proximity Analysis

Used the Near tool to calculate distance (km) from each well to nearest cropland and ranchland, enabling spatial exposure analysis.

Part 5: Pesticide Use Integration

Standardized county names and joined pesticide rates to the well dataset to allow exploration of fertilizer impact on contamination.

Part 6: Linear Regression Analysis

Used OLS (Ordinary Least Squares) to model nitrate concentrations as a function of:

- Percent cropland and ranchland in basin
- Distance to cropland and ranchland
- (Future work) Pesticide application rates