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Dynamics and sources of colloids in shallow groundwater in lowland wells and fracture flow in sloping farmland

Zhang, Wei, Cheng, Jian-Hua, Xian, Qing-Song, Cui, Jun-Fang, Tang, Xiang-Yu, Wang, Gen-Xu
Water research 2019 v.156 pp. 252-263
agricultural land, agricultural wastes, agricultural watersheds, aquifers, calcium carbonate, carbon, cations, colloids, dry season, groundwater, heavy metals, hydrochemistry, magnesium, mineral content, monitoring, organic matter, pH, pesticides, rain, regression analysis, soil, stable isotopes, structural equation modeling, temporal variation, water temperature, wells, wet season
Field-scale studies of natural colloid mobilization and transport in finely fractured aquifer as well as the source identification of groundwater colloids are of great importance to the safety of shallow groundwater. In this study, the daily monitoring of fracture flow from a sloping farmland plot and the biweekly monitoring of three lowland shallow wells within the same catchment were carried out simultaneously in 2013. The effects of physicochemical perturbations on groundwater colloid dynamics were explored in detail using partial redundancy analysis, structural equation modeling, Pearson correlation and multi-linear regression analyses. The characterization and source identification of groundwater colloids were addressed via multiple parameters. The daily colloid concentration in the fracture flow varied between 0.54 and 31.90 mg/L (1.64 mg/L on average). Unique periods of high colloid concentration (5.59 mg/L on average) occurred during the initially generated flow following the dry season. In comparison, a narrower colloid concentration range of 0.24–11.66 mg/L was observed in the lowland shallow wells, with a smaller temporal variation than that of the fracture flow. A low percentage (2.4–7.0%) of colloids and a high percentage (47.7–92.0%) of coarse particles (2–10 μm) were present in the lowland well water. Hydraulic perturbation by rainwater infiltration in the sloping farmland was the dominant mechanism for colloid mobilization in general; this effect retreated to secondary importance behind chemical perturbations (pH, Mg2+ and DOC) at low flow discharges (<1.3 L/min). In contrast, water chemistry (e.g., EC, cations and DOC concentrations) exhibited a major effect on colloid dynamics in the water of the lowland wells, except for the extremely high-salinity water of one well, in which water temperature showed a negative dominant influence on colloid stability. The combined use of multiple parameters (e.g., mineral composition and organic matter, calcium carbonate and δ13C contents) traced groundwater colloids to the shallow soil in the upper farmlands. It is strongly advised that in finely fractured aquifers within agricultural catchments, not only the small colloids but also the coarse particles in the size range of 2–10 μm should be monitored in case of colloid-associated contamination from agricultural wastes e.g., N, P, pesticides and/or heavy metals, especially at the early stages of the rainy seasons.