Changes in characteristics of soils irrigated with processing wastewater from three New Zealand dairy factories

Abstract

Application to land is the preferred method for the treatment of wastewaters in New Zealand. For land treatment to be effective, it is essential that the soils can accept the volumes of wastewater applied and degrade or store the constituents in the wastewater. We report on 14 soil chemical, biochemical and physical characteristics of soils (0–10 cm depth) used for wastewater treatment at the Fonterra dairy factories at Hautapu, Lichfield and Edgecumbe in the North Island of New Zealand. The soils are under grazed pasture for dairying and receive wastewater by spray irrigation. The soils were monitored approximately every 2 years between 1995 and 2005 and at the end of monitoring had been under irrigation for 10–26 years. Matched, non-irrigated pasture soils on adjacent dairy farms were sampled for comparison. The wastewater composition from the three factories differed, reflecting the products manufactured. Loadings were greatest at the Hautapu factory, which also had the longest history of irrigation (26 years). At all three sites, the physical characteristics of irrigated soils were very similar to their non-irrigated comparisons. A consistent trend was for microbial mass and activity, and particularly nitrogen (N) turnover, to be markedly greater on the irrigated soils. The C (carbon) : N ratios of irrigated and non-irrigated soils at Lichfield and Edgecumbe were similar, but at Hautapu the C : N ratio of irrigated soil was 8.3 and significantly (P < 0.05) lower than non-irrigated soil (11.1), suggesting little further capacity to store additional N as organic matter. Irrigation tended to increase the soil pH at all sites to above neutral even though the wastewater was acidic. We consider that the characteristics of irrigated soils at Edgecumbe and Lichfield factories are generally satisfactory. Fonterra is continuing to reduce loadings in both composition and volumes of wastewater irrigated.