Imprint of agriculture on major element hydrogeochemistry in a small, semi-arid catchment

Michael Craig Shaljian

While there has been much study of the geochemistry of natural waters, very little work has been done to investigate the influence of management on major element solution chemistry in agricultural catchments. Mineral-derived nutrient cycling in agroecosystems is also poorly understood and hydrologic effluxes of these nutrients represent a substantial loss vector that has seldom been quantified. Here I examine concentrations of base cations, anions and silica to characterize the water chemistry of drainage from an agricultural catchment, investigate concentration-discharge relationships, estimate solute effluxes and infer potential solute sources. Artificial drain discharge and concentrations of major elements in that discharge were measured at the Cook Agronomy Farm (CAF) during the 2015 and 2016 water years (WYs). The main objectives of this study were to: (1) conduct multi-year monitoring of an artificial drain and investigate seasonal and inter-annual variations in hydrogeochemistry; (2) identify the drivers of solute export from an agricultural catchment; (3) develop a conceptual model that partitions weathered and exchangeable base cation sources, yielding a net cation denudation rate for the catchment. Artificial drainage from the CAF is calcium-bicarbonate-nitrate dominant, with bicarbonate as the dominant anion during the low-discharge seasons of fall and summer and nitrate as the dominant anion during the high-discharge period from early winter to mid-spring. Most solutes exhibited minor to moderate dilution with increased discharge, with the exception of K+, NO3- and SO42-. Concentrations varied no more than 4X while flows varied 130X. On an inter-annual timescale, solutes were observed to behave nearly chemostatically, with discharge-normalized effluxes for WYs 2015 and 2016 differing only negligibly for all solutes except Na+ and Cl-. The stoichiometry of typical weathering reactions of silicate minerals does not agree with observed rates of release of base cations and silica, as base cation losses are ~2X greater than silica losses on a molar basis. This implies that mineral weathering is not the sole source of nutrient-derived cations and soil exchange must also contribute to losses. Discharge-normalized effluxes of base cations were an order of magnitude greater than those observed in unmanaged North American catchments on silicate terranes, further implicating exchange-driven losses.

Imprint of agriculture on major element hydrogeochemistry in a small, semi-arid catchment

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