Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand
Agriculture
; Biochemical oxygen demand
; Catchments
; Chemical analysis
; Cluster analysis
; Contamination
; Dissolved oxygen
; Eutrophication
; Geology
; Groundwater
; Groundwater flow
; Groundwater geochemistry
; Groundwater pollution
; Hierarchical systems
; Hydrochemistry
; Lakes
; Land use
; Nitrates
; Nuclear weapons
; Nutrients
; Rivers
; Runoff
; Tritium
; Volcanoes
; Water quality
; Agricultural activities
; Anthropogenic contaminants
; Ground water discharge
; Groundwater chemistry
; Hierarchical cluster analysis
; Microbial denitrification
; Nuclear weapons testing
; Nutrient contamination
; Lake pollution
; anthropogenic source
; catchment
; discharge
; eutrophication
; hydrochemistry
; lacustrine deposit
; lake pollution
; nuclear weapons testing
; suspended load
; water quality
; Bay of Plenty
; Hamilton [Waikato]
; Hamurana Stream
; Lake Rotoroa
; New Zealand
; North Island
; Waikato
; algae
英文摘要:
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.
The groundwater in the Rotorua catchment, once it has passed through the soil zone, shows no further decrease in dissolved oxygen, indicating an absence of bioavailable electron donors along flow paths that could facilitate microbial denitrification reactions. Nitrate from land-use activities that leaches out of the root zone of agricultural land into the deeper part of the groundwater system must be expected to travel with the groundwater to the lake.
GNS Science, P.O. Box 30368, Lower Hutt, New Zealand; Hawke's Bay Regional Council, Private Bag 6006, Napier, New Zealand; Department of Applied Geology, Georg-August-Universität Göttingen, Goldschmidtstr. 3, Göttingen, Germany; GNS Science, Private Bag 2000, Taupo, New Zealand
Recommended Citation:
Morgenstern U,, Daughney C,J,et al. Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand[J]. Hydrology and Earth System Sciences,2015-01-01,19(2)