The impact of on-site wastewater from high density cluster developments on groundwater quality

• Published in: Journal of contaminant hydrology 2015-11, Vol.182, p.36-50
• Main Authors: Morrissey, P.J., Johnston, P.M., Gill, L.W.
• Format: Article
• Language: English
• Subjects: Calibration; Cluster developments; Clusters; Computer simulation; Contaminants; Density; Environment; Environmental Monitoring - methods; Ground-water quality; Groundwater - chemistry; Groundwater - microbiology; High density; Ireland; Mathematical models; Modelling; Models, Theoretical; On-site wastewater; Septic tank; Vadose zone; Waste Disposal, Fluid - methods; Waste Water; Water Pollutants, Chemical - analysis; Water Quality
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/j.jconhyd.2015.07.008

The net impact on groundwater quality from high density clusters of unsewered housing across a range of hydro(geo)logical settings has been assessed. Four separate cluster development sites were selected, each representative of different aquifer vulnerability categories. Groundwater samples were collected on a monthly basis over a two year period for chemical and microbiological analysis from nested multi-horizon sampling boreholes upstream and downstream of the study sites. The field results showed no statistically significant difference between upstream and downstream water quality at any of the study areas, although there were higher breakthroughs in contaminants in the High and Extreme vulnerability sites linked to high intensity rainfall events; these however, could not be directly attributed to on-site effluent. Linked numerical models were then built for each site using HYDRUS 2D to simulate the attenuation of contaminants through the unsaturated zone from which the resulting hydraulic and contaminant fluxes at the water table were used as inputs into MODFLOW MT3D models to simulate the groundwater flows. The results of the simulations confirmed the field observations at each site, indicating that the existing clustered on-site wastewater discharges would only cause limited and very localised impacts on groundwater quality, with contaminant loads being quickly dispersed and diluted downstream due to the relatively high groundwater flow rates. Further simulations were then carried out using the calibrated models to assess the impact of increasing cluster densities revealing little impact at any of the study locations up to a density of 6units/ha with the exception of the Extreme vulnerability site. •Impact on groundwater quality from high density cluster developments monitored•No significant impact from cluster systems on monitored downstream groundwater•Linked models of pollutant transport through unsaturated zone and groundwater•Models corroborate field study results.•Little potential contamination from clusters up to a density of 6units/ha