Septic-derived nutrient contamination of shallow groundwater in Lake Tarawera: Extent, fate, and ecological consequences

Abstract

Onsite wastewater treatment systems (OWTS) are important point sources of nitrogen and phosphorus pollution to lakes, with nutrients transported via groundwater to the nearshore zone where they can stimulate algal growth and degrade littoral ecosystems. At Lake Tarawera (Bay of Plenty, New Zealand), OWTS are estimated to contribute 3–5 % to the annual nutrient load, prompting implementation of a sewage reticulation scheme of the residential area along the lake’s western margin. However, the spatial distribution, subsurface transport, and ecological consequences of septic-derived nutrients remain poorly quantified. This study assessed shallow groundwater contamination within the urbanised western margin, examined groundwater–littoral transport pathways, and evaluated potential effects on benthic primary producers.

Groundwater was sampled monthly from 21 piezometers along upslope and downslope transects between April 2023 and March 2025 and analysed for nitrate-N, nitrite-N, ammoniacal-N, and dissolved reactive phosphorus (DRP). Groundwater connectivity to the lake was assessed by relating antecedent rainfall to groundwater flux into the littoral zone, measured using 24-hour benthic chamber deployments. Stable isotope ratios of nitrate (δ15N and δ18O) in groundwater and pore water, alongside δ15N and δ13C in macrophyte, benthic algal, and suspended particulate organic matter (SPOM) tissue, were used to trace septic-derived nitrogen transport. Benthic gross primary production (GPP) was estimated seasonally at six nearshore sites using 1.5-hour benthic chamber incubations to assess potential changes in littoral metabolism.

Results revealed localised areas of elevated nutrient concentrations within the urban zone, with high maximum values observed for ammoniacal-N (61 mg L-1), nitrate-N (7.4 mg L-1), DRP (8.7 mg L-1), and nitrite-N (0.91 mg L-1), consistent with previously reported septic contamination ranges and higher than those measured in a prior study at the lake. No consistent seasonal or annual patterns were observed. Enriched δ15N values were observed in both upslope groundwater (5.50 ± 3.18 ‰) and littoral pore water (6.51 ± 3.43 ‰), indicating wastewater-derived nitrogen inputs to the adjacent littoral zone. However, low δ15N values in SPOM, macrophytes, and benthic algae (1.48 ± 0.83 ‰), suggested limited assimilation by primary producers. Sites exhibiting dense epiphytic algal growths during spring and summer displayed the highest benthic GPP rates (maximum = 296 mg O₂ m² h⁻¹). Although vegetation biomass explained the most variation in GPP (19 %), multiple lines of evidence suggest that upslope groundwater nutrient enrichment may have promoted short-term epiphytic algal growth at certain sites under favourable growing conditions.

This study provides integrated evidence linking septic-derived nutrient transport through groundwater to altered benthic community structure in Lake Tarawera. Although measurable impacts on primary producer assimilation were limited, substantial nutrient loading to shallow groundwater and the nearshore zone was evident. Periphyton responses appear to be a sensitive indicator of early nutrient pressure preceding broader metabolic changes. Continued inputs may increase the risk of shifts towards greater algal dominance, underscoring the need to address OWTS sources proactively. Lake Tarawera serves as a case study of a widespread and often underestimated issue of nutrient contamination from OWTS across New Zealand, emphasising the importance of strengthened monitoring and management to help combat further freshwater degradation.