Modelling the impact of sewage reticulation in the Lake Tarawera catchment
Dada, A. C., McBride, C. G., Verburg, P., & Hamilton, D. P. (2016). Modelling the impact of sewage reticulation in the Lake Tarawera catchment (ERI report). Hamilton, New Zealand: Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato.
Permanent Research Commons link: https://hdl.handle.net/10289/12466
In order to inform efforts being considered for the management of catchment sources of nutrient pollution into Lake Tarawera, a study was commissioned by the Lake Tarawera Ratepayers Association. This study, conducted by the University of Waikato, was specifically to assess the potential impacts of sewage reticulation on lake water quality. Analyses of available data indicate that Lake Tarawera is regionally unique in that the majority of its phosphorus (P) load and a substantial portion of its nitrogen (N) load appear to be derived from a combination of ‘tributary’ lakes and geothermal sources. This may help explain the relatively high P concentrations and low N: P ratio in the lake. Waste water from septic systems contribute 3.3% (2.9t N y⁻¹) of the total N load and 2.7% (0.29 t P y⁻¹) of the total P load to Lake Tarawera This source of nutrients represents approximately 15% of the ‘manageable’ load from land use in the surface catchment. Mass balance models are often used to estimate catchment external load corresponding to desired in-lake nutrient concentrations or trophic state (assuming internal loading is negligible). In this study we adopted a reverse approach, using mass balance models to estimate the response of in-lake concentrations to changes in external load (namely, with and without sewage reticulation). Retention of phosphorus was estimated using the method of Vollenweider (1976) and retention of nitrogen was estimated after Harrison et al. (2009), with external load retention coefficients¹ of 0.73 and 0.72 for P and N, respectively. Improperly treated domestic sewage from lakeside properties may contain pathogens including bacteria, viruses, protozoa and helminths (intestinal worms and worm-like parasites). Historical measurements of E.coli concentrations from tap water samples collected from a number of sites around the lake between 1991 and 2015 were thus analysed, as many residential properties draw water directly from the lake and/or from closely linked groundwater. Given that microbiological noncompliance of drinking water samples was prevalent at most (6 out of 10) of the sampling locations considered in this analysis, authorities may need to look into more frequent testing and to reduce potential sources of faecal contamination within the catchment, including failed septic tanks, to improve public safety. In light of the current study, wastewater reticulation could make a significant contribution to mitigating public health risks associated with poorly performing on-site treatment systems. From a public health perspective, this study also highlights the need for efforts aimed at investigating and curbing potential sources of faecal contamination of drinking water sources within the catchment. This could include the commissioning of a microbial source tracking study that uses appropriate genetic markers to discriminate among the possible sources of faecal pollution within tap water used for domestic purposes in the Lake Tarawera catchment. These efforts ideally would be put in place before and after the execution of any sewage management intervention to initially establish the extent of human faecal contamination from faulty septic tanks and ultimately, to confirm that the sewage management efforts put in place are effective. As part of Lake Tarawera water quality management plan, a wastewater reticulation system will reduce nutrient loading by 3 to 5%. Modelling of the reticulation of wastewater nutrient loads revealed that the impact on lake water nutrient concentrations is minor, as wastewater represents a small component of overall N and P loads to Lake Tarawera. Nevertheless, much of the load to the lake is from sources that are difficult to mitigate (e.g. geothermal, other lakes). Further, time lags in response of the nutrient loads to the lake may be lower due to the proximity of current wastewater inputs to the lake shore (as opposed to diffuse inputs from land use on the upper reaches of the catchment). Therefore, implementation of a reticulated sewage system as part of an overall plan to reduce loading of P and N into Lake Tarawera may be a desirable management initiative as it could make a contribution to improving manageable sources of nutrients from the lake catchment. In addition, a reticulated sewage system will reduce public health risks associated with poorly performing on-site treatment systems. From a public health perspective, efforts such as further intensive sampling for faecal coliforms in drinking water, as well as the commissioning of microbial source tracking studies to identify faulty septic tanks is recommended. These investigations will also provide information on the performance of sewage management efforts (and optimisation) to reduce the discharge of pathogenic contaminants into the lake. This report has not made recommendations on the costs or value for money that sewage reticulation would entail. These considerations will be essential to complement our environmental investigation and underpin considerations of a reticulated sewage system for Lake Tarawera.
Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato
© 2016 copyright with the authors.