The study investigated the effects of 26 years of effluent irrigation on chemical and bacteriological quality of shallow (<3.0 m) groundwater. Annual loading rates for N and P exceeded pasture requirements, while trace metals were either lower or higher than guideline limits. Effluent irrigation removed TN (44–71%), TP (80%), Cr (96%) and coliform bacteria (87–99.9%) while Zn, Cu and Cd removal was negligible probably due to their enhanced mobility. Analysis of groundwater samples from effluent-irrigated and non-irrigated control sites showed that effluent irrigation increased the levels of all measured parameters compared to the control. Average groundwater quality parameters from effluent-irrigated sites compared to the control were: pH (6.1 vs. 5.7), EC (0.71 vs. 0.53 dS m−1), concentrations (mg L−1) for TP (2.3 vs. 0.3), DP (1.0 vs. 0.1), TN (15.1 vs. 2.5), NH4–N (2.6 vs. 0.5), NO3–N (4.1 vs. 1.3), Zn (0.4 vs. 0.05), Cu (0.13 vs. 0.02), Cd (0.05 vs. 0.01) and Cr (0.06 vs. 0.03). Across effluent-irrigated sites, FC and TC were 25 and 288 cfu/100 ml, respectively, versus nil for the control. Overall, effluent irrigation led to groundwater contamination by N, P, trace metals and coliform bacteria, which could threaten the long-term sustainability of the practice.

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