Long‐Term Quality of Shallow Ground Water at Irrigated Sites

Abstract

Areal, vertical, and temporal differences in salinity characteristics of shallow, unconfined ground water were related to irrigation, cropping practices, and surficial geology. Nine wells were monitored monthly for a period of up to 20 years for electrical conductivity (EC), Ca+MgandNa content, and the sodium adsorption ratio (SAR) was calculated from the ionic concentrations. In the last year of the study, the samples were occasionally analyzed also for K, Mg, and major anion contents. The ionic composition of the ground water was dominated by Ca+MgandSO4. Spatial, vertical, and seasonal variabilities were affected by cropping practice and depth to glacial till. Ground‐water salinity (EC) near the water table was lower under annual than perennial cropping and had a higher mean EC and higher mean seasonal EC in soils having till at less than 1.5 m from the surface than in those with till at depths greater than 1.5 m. Irrigation applications and precipitation events caused dilution of the poor‐quality ground water resulting in vertical water‐quality gradients in the top part of the aquifer. The vertical extent of the gradient depended mainly on the nature and thickness of the vadose zone. Temporal fluctuations were greater during the irrigation season than the off‐season. Ground‐water salinity near the water table since initiation of the study has trended toward higher EC values, probably due to improvements in irrigation efficiency.