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| Titel |
Using groundwater age and hydrochemistry to understand sources and dynamics of nutrient contamination through the catchment into Lake Rotorua, New Zealand |
| VerfasserIn |
U. Morgenstern, C. J. Daughney, G. Leonard, D. Gordon, F. M. Donath, R. Reeves |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 19, no. 2 ; Nr. 19, no. 2 (2015-02-05), S.803-822 |
| Datensatznummer |
250120623
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| Publikation (Nr.) |
 copernicus.org/hess-19-803-2015.pdf |
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| Zusammenfassung |
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.
The old age and the highly mixed nature of the water discharges imply a very
slow and lagged response of the streams and the lake to anthropogenic
contaminants in the catchment, such as nitrate. Using the age distribution
as deduced from tritium time series data measured in the stream discharges
into the lake allows prediction of future nutrient loads from historic
land-use activities 50 years ago. For Hamurana Stream, the largest stream to
Lake Rotorua, it takes more than a hundred years for the
groundwater-dominated stream discharge to adjust to changes in land-use
activities. About half of the currently discharging water is still pristine
old water, and after this old water is completely displaced by water
affected by land use, the nitrogen load of Hamurana Stream will
approximately double. These timescales apply to activities that cause
contamination, but also to remediation action. |
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