The impact of land use on 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 on such impacts by combining nitrate measurements from 6016 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 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 m of a well is associated with an 11.6% increase in nitrate concentrations a decade later, while the same increase in urban developments contributes about a 10% increase. When conditioning on initial nitrate measurements, the impact of nearby land use attenuates while initial concentrations explain a large share of future variation in concentrations, demonstrating the persistent nature of nitrates in groundwater. A calculation based on our regression estimates implies that replacing high-nitrogen with low-nitrogen crops around sample wells would achieve a 4.6% reduction in nitrate concentrations, saving municipal water systems $25 million annually. We evaluate the opportunity cost of such crop substitution to be large; however, targeting only the crops with the highest propensity to leach nitrates easily passes a cost-benefit test.