Denitrification bioreactor work in waituna lagoon catchment , southland

Subsurface drainage of agricultural land provides multiple “conduits” that potentially facilitate transport of contaminants from field to stream. In addition to increasing the efficiency of transport, these “conduits” may also allow contaminants to bypass natural attenuation zones and processes. Adoption of tile or mole drainage has contributed to the trend of increasing nitrate-nitrogen loads observed in many rural streams. Despite creating the potential to degrade downstream water quality, relatively few long-term, field-scale trials have been undertaken to quantify the hydraulic and contaminant load from these drainage systems. We quantified the mass load of nitrogenous material derived from a representative subsurface drain, and the reduction in nitrate-nitrogen provided by a woodchip filter during a 15-month trial. The tile drain delivers seasonally-varying concentrations and mass of contaminants, driven primarily by rainfall. The annual median concentrations of nitrate-nitrogen entering the filter (i.e. discharged from the tile drain), and discharged from the filter were 2190 μg/L and 421.5 μg/L, respectively. The median daily load was reduced from 47.1 g/d to 2.9 g/d (a reduction of 93.8 percent), and the median nitrate-nitrogen yield from the field was reduced from 1.83 kg/ha/yr to 0.11 kg/ha/yr. Efficacy of the woodchip treatment filter was strongly dependent on hydraulic retention time, and to a lesser extent, temperature. These dependencies were evident as seasonally varying treatment efficacy. These factors should be considered before modifying the design of a woodchip filter to enhance efficacy. For example, if seasonally large hydraulic loading determines treatment efficacy during autumn and winter, it may be possible to improve performance by buffering the flow (e.g. by incorporating a buffer chamber into the filter design), or by temporarily retaining some of the drainage water in the landscape, within the drains themselves. Another approach could include a bypass system to limit inflow to the filter once a defined flow threshold was achieved. Several of these and other approaches may be combined to accommodate site-specific considerations (e.g. soil, gradient and farming system factors). It is also conceivable that seasonally varying treatment efficacy may be consistent with the seasonality of the receiving environment and with water quality improvement goals.