Fate and transport of nitrogen from wastewater irrigated to land adjacent to a sensitive harbour
Annual Conference
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As part of the reconsenting of the Omaha Wastewater Treatment Plant (WWTP) land discharge system, Watercare Services Ltd commissioned a study to understand the fate and transport of nutrients and the effects on the environment. The discharge of the treated wastewater is to two irrigation blocks, one on Watercare land (eucalypt and native bush) and the other being the Omaha Golf Course. While the discharge is to land, ultimately the water is transported through the groundwater to the Whangateau Harbour, which is a high quality waterway. Understanding the fate and transport of nutrients to maintain the harbour water quality was an important aspect for the stakeholders of this project.
We focussed on nitrogen transformations in soil beneath the irrigation sites. We measured a range of biochemical rates in soil cores to understand nitrate loss (denitrification) and nitrate production (nitrification). We also made additional chemical measurements to determine the inorganic nitrogen concentrations in pore water, the amount of readily mineralizable carbon, and redox potential (Eh) at the boundary with the groundwater table. We also commissioned other experts to estimate nitrogen uptake and immobilization in the eucalypt stand, and golf course, respectively.
A parallel study used a variety of geophysical measurements on the strata between the irrigation sites and the harbour to construct a model of groundwater flows and travel times. We used their soil-water leaching loss algorithms together with rate measurements derived from our biochemical testing and estimates of plant uptake/immobilization to construct a stochastic model of nitrogen loss from irrigated wastewater within the unsaturated soil profile. We also made estimates of likely N losses in saturated organic strata using an analytical solution that predicted N concentration as a function of conservative rate constants and residence time.
Our results showed that the peat soils on the Jones Rd sites had measurable in-situ denitrification throughout the soil profile. Denitrifying enzyme activity (DEA, indicator of size of denitrifying populations) was highest in the surface layers and declined with depth. On the golf course sites, high DEA and measurable in-situ denitrification activity occurred within the surface turf layer but declined to trace levels in the sand substrate beneath. Redox potential (Eh) at the boundary with the groundwater table was generally consistent with the published range of values associated with denitrification.
The modelling demonstrated that leached loads and concentrations were much higher for the golf course than the Jones Rd sites. This was due to a combination of high loads (in summer) and shorter residence times within a 15-30 cm thick ‘active’ biological zone. Using conservative assumption for rate constants in the saturated zone (derived from measurements), the model predicted only trace concentrations (<0.1 g N/m3) would enter the harbour. A sensitivity analysis of residence times showed that 200 days residence through saturated organic layers, compared with 15-40 years travel time predicted in the groundwater study, was sufficient to ensure that effectively no nitrogen sourced from the WWTP would enter the harbour. The study was accepted by the regulator and a 35-year consent was granted.