Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand

Abstract

Abstract. The water quality of Lake Rotorua has steadily declined over the past 50 years despite mitigation efforts over recent decades. Delayed response of the groundwater discharges to historic land-use intensification 50 years ago was the reason suggested by early tritium measurements, which indicated large transit times through the groundwater system. We use the isotopic and chemistry signature of the groundwater for detailed understanding of the origin, fate, flow pathways, lag times and future loads of contaminants. A unique set of high-quality tritium data over more than four decades, encompassing the time when the tritium spike from nuclear weapons testing moved through the groundwater system, allows us to determine detailed age distribution parameters of the water discharging into Lake Rotorua.The Rotorua volcanic groundwater system is complicated due to the highly complex geology that has evolved through volcanic activity. Vertical and steeply inclined geological contacts preclude a simple flow model. The extent of the Lake Rotorua groundwater catchment is difficult to establish due to the deep water table in large areas, combined with inhomogeneous groundwater flow patterns.Hierarchical cluster analysis of the water chemistry parameters provided evidence of the recharge source of the large springs near the lake shore, with discharge from the Mamaku ignimbrite through lake sediment layers. Groundwater chemistry and age data show clearly the source of nutrients that cause lake eutrophication, nitrate from agricultural activities and phosphate from geologic sources. With a naturally high phosphate load reaching the lake continuously via all streams, the only effective way to limit algae blooms and improve lake water quality in such environments is by limiting the nitrate load.

Comment on "Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand" by Morgenstern et al. (2015)

“…Natural loads correspond to baseline conditions prior to human presence in the catchment and were estimated as the sum of baseline groundwater loads and baseline surface loads. Baseline groundwater loads were estimated using the authors' results, while baseline surface loads were estimated using baseline concentrations estimated by McDowell et al (2013). For the whole lake catchment, the study estimated that 48 % of the total P load and 22 % of the dissolved reactive P load originate from anthropogenic sources.…”

“…Consequently, at the national scale, streams draining catchments with such acidic volcanic geology have higher baseline (i.e. natural) PO 3− 4 concentrations than comparable streams that drain different geologies (McDowell et al, 2013). The authors demonstrate this occurrence very convincingly for the Lake Rotorua catchment in Fig.…”

Application of tritium in precipitation and baseflow in Japan: a case study of groundwater transit times and storage in Hokkaido watersheds

“…Knowing groundwater travel times allows us to pinpoint possible sources of groundwater pollution from agricultural activities, while estimates of groundwater volumes in the subsurface are needed for sustainable management of water resources in many countries (Granneman et al, 2000;McMahon et al, 2010;Gusyev et al, 2011Gusyev et al, , 2012Stewart et al, 2012;Morgenstern et al, 2015). In Japan, there is a need for a robust and quick approach to quantify the subsurface groundwater volume as an important component of the water cycle due to the recently enacted Water Cycle Basic Law in March 2014 (Tanaka, 2014).…”

“…In the Southern Hemisphere, tritium measurements in river water have been commonly used to understand groundwater dynamics by determining groundwater transit time distributions and by constraining groundwater flow and transport models (Stewart et al, 2007;Gusyev et al, 2013Gusyev et al, , 2014Morgenstern et al, 2010Morgenstern et al, , 2015Cartwright and Morgenstern, 2015;Duvert et al, 2016). Tritium is a part of the water molecule and migrates through the water cycle while being inactive except for radioactive decay.…”

“…Therefore, a common practice is to utilise numerical models with simplified representations of the complex groundwater dynamics for rainfall-runoff simulation in river catchments. For example, a distributed hydrologic model BTOP, which has been applied globally and in many river basins for detailed flood and drought hazard quantification (Gusyev et al, 2016;Navarathinam et al, 2015;Nawai et al, 2015), is used to simulate groundwater flow components using the exponential mixing model (EMM) with mean travel distance of groundwater flow (Takeuchi et al, 2008). At the river basin scale, a simple and yet robust tracer such as tritium ( 3 H) is needed to characterise the groundwater bodies as they are drained by surface water features.…”