IMPACT OF IRRIGATED HORTICULTURE ON NITRATE CONCENTRATIONS IN GROUNDWATER

Irrigated horticulture, and particularly market garden areas located on sandy soils in the Swan Coastal Plain, Western Australia, were examined as sources of nitrates in groundwater and root zone drainage. Data were collected from lysimeter-equipped sites, and shallow (5-15 m) and deeper (20-60 m) irrigation well networks positioned to sample these areas. The NO3-N concentrations in drainage below the root zone ranged from 71 to 209 mg 1-1 and approached potential N losses from market gardens as computed by N mass balance. Overall, the nitrate concentrations in groundwater were substantially less than the potential, but still high. NO3-N concentrations in the shallow well network ranged from 9 to 79.9 mg 1-1 with a mean of 31.2 mg 1-1. NO3-N concentrations were similar for about one-third of the deeper irrigation wells, but were much less for most of the remaining deeper wells, ranging from 0 to 5 mg 1-1. Denitrification and dilution appear as the major processes causing these much lower nitrate concentrations. Among the several non-nitrogen chemical indices tested to provide an independent measure of horticultural impact and a reference useful for interpreting the nitrate results, the sulphate to chloride ratio and sulphate concentration appeared best. © 1990.