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Tracing the movement of irrigated effluent

Abstract

Three microbial tracers – Escherichia coli J6-2, a somatic coliphage (ØESR1) and endospores of Bacillus subtilis var. niger NCIB 8649 tracer strain JHI – were added to effluent flood irrigated onto border dyke strips at a sewage treatment plant near Christchurch, New Zealand. All three tracers, and three effluent indicators – faecal coliforms, F-RNA phages, and chloride – were recovered in a bore, approximately 100 m downstream. A simple spatial model was applied to the breakthrough curves (BTCs) in the bore, using a series of hypothetical “entry points” in the strips. This analysis indicated effluent transport velocities through the 16.8 m deep vadose zone of between 15.7 and 39.2 m hr− 1. The shapes of the BTCs for the microorganisms and chloride were very different, suggesting that they reached the groundwater table via two pathways: – both underwent rapid transport to the groundwater though macropores, but chloride also underwent far slower (matrix) transport though micropores. The BTC shapes also suggested transport velocities in the vadose zone of E. coli J6-2 > B. subtilis JH1 endospores > phage ØESR1, which is consistent with the theory of pore size exclusion, based on particle size. Reductions in microbial concentrations were ≈100 times greater than for chloride, and occurred rapidly, suggesting that up to 99% of the microorganisms underwent early exclusion from macropore flow and were removed during matrix flow. Nevertheless, the results show that substantial numbers of bacteria and viruses will still reach the groundwater through macropores beneath effluent irrigation schemes located on alluvial gravel formations.

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