The Impact of Land Use of Water Quality: Evidence from California Wells

Nitrate pollution threatens human health and ecosystems in many regions of the world. Although scientists agree that nitrogen compounds from human activity, notably agriculture, enter groundwater systems, empirical estimates of the impacts of land use on nitrate concentrations in well water are still lacking. We provide evidence of such impacts by combining nitrate concentration measurements from 6,016 groundwater wells with remotely sensed California land use data from 2007–2023. We categorize agricultural land uses according to crops’ propensities to leach nitrogen and further consider high- and low-density urban development, in addition to undeveloped land—the default land use. Results show that a 10 percentage point increase in the share of land used to grow high-nitrogen crops within 500 meters of a well is associated with a 12% increase in nitrate concentrations a decade later. The same increase in low-intensity urban development is associated with a 10% increase. Local cattle populations also contribute to nitrate pollution. When conditioning on initial nitrate measurements, the impact of nearby land use on future nitrate concentrations attenuates and is imprecisely estimated, demonstrating the legacy nature of nitrate leaching into groundwater. A calculation based on our regression estimates further implies that replacing high-nitrogen with low-nitrogen crops around our sample of wells would achieve a 4.8% reduction in nitrate concentrations, saving municipal water systems approximately $25 million annually.