Prerequisites for incorporating groundwater assimilative capacity as a legitimate treatment component of a land based waste treatment system

Abstract

Nitrogen assimilation processes that occur when effluent is applied onto a land based waste treatment (LBWT) site are not necessarily restricted to the root zone. There is growing recognition that denitrification can significantly reduce the mass of nitrate (NO₃-N) in some groundwater systems before the groundwater discharges into surface water bodies. Taking this natural assimilative capacity into account when making decisions on the sustainable nitrogen loadings to a LBWT system can significantly change the required land area, and hence the economics of the operation of a land (and groundwater) based treatment system. Where groundwater denitrification occurs, incorporating it in a LBWT system would have substantial benefits, as groundwater denitrification can remove large NO₃-N loads. This is evident from the very low NO₃-N concentrations generally observed in reduced groundwater systems even where land surface recharge is high in NO₃-N. Additionally, laboratory studies have shown that the denitrification capacity typically lies in the order of tens to hundreds of kg N/ha/yr. Remediation of any detrimental effects resulting from insufficient performance of such a proposed treatment system would be difficult and costly. Hence, substantial investigation efforts are required to defensibly demonstrate that the groundwater system has the on-going capability to sustain the removal of NO₃-N over the design life of the treatment system. Denitrification converts NO₃-N to gaseous forms of N. In contrast to unsaturated zone denitrification, where incomplete denitrification can result in substantial emissions of the greenhouse gas nitrous oxide (N₂O), complete denitrification to inert dinitrogen (N₂) prevails under the more stable redox conditions of reduced groundwater zones. For denitrification to occur, four requirements must be met. Apart from NO₃-N being present, there needs to be oxygen-depleted conditions, a suitable electron donor, and microbes with the metabolic capacity for denitrification. NO₃-N and suitable microbes are considered ubiquitous under agricultural land use. Accordingly, the occurrence of denitrification at a particular location is largely determined by the existence of oxygen-depleted conditions (< 2 mg/L dissolved oxygen (DO)) and the availability of electron donors. For heterotrophic denitrification the electron donor is organic matter, while for autotrophic denitrification this role is met by reduced inorganic iron and sulphur compounds (e.g. pyrite). Through a combination of field based measurements using hydrochemical analyses, isotopic analyses, excess N₂ determinations, and microcosm studies combined with laboratory incubations it has been ascertained that denitrification occurs in some groundwater systems in the Waikato region. Particulate organic matter residing in the groundwater zone has been identified as the main electron donor in the upper part of the groundwater, but additional contributions of NO₃-N reduction fuelled by inorganic electron donors in deeper parts of the groundwater system cannot be discounted. Robust measurements allowing denitrification to be identified locally, combined with defensible groundwater flux and velocity monitoring, are necessary to quantify the effects of denitrification on the NO₃-N fluxes beneath a LBWT site. If these requirements can be achieved, then acceptable effluent N loading rates to a site can be determined that take the quantified groundwater assimilative capacity into account. The hydraulic loading to the site will also be concurrently increased, so it is necessary to show that the higher hydraulic loading is; agronomically sustainable, not creating excess runoff, and not diminishing the groundwater assimilation processes.