『Summary
Widespread sulphidic deposits have accumulated in tropical coastal
floodplains throughout the world. Sulphidic soils oxidize when
floodplains are drained for urban and agricultural development.
As a result, large amounts of sulphuric acid may be released to
nearby waterways. Macropores may create excellent conditions for
groundwater flow in coastal acid sulphate soils (CASS). An automated
radon (222Rn) measurement system was used to quantify
groundwater inputs into a tidally-dominated estuary that is known
to be influenced by acid discharges from CASS (Richmond River
Estuary, Australia). A high resolution radon survey along a 120-km
long segment of the tidal river identified two areas of preferential
groundwater inputs. Intensive time series measurements in one
of those areas (the Tuckean Broadwater) demonstrated that groundwater
inputs are highly variable over hourly and seasonal time scales
and inversely related to surface water pH. Elevated radon concentrations
(up to 12 dpm/L) and low pH (as low as 3.3) were observed in surface
waters at low tide a few weeks after a large rain event. These
results demonstrate that acidic waters are entering the estuary
via tidally-modulated groundwater flow pathways. Groundwater discharge
rates into drains in the Tuckean Swamp were estimated from a dual-assumption
radon mass balance to be 0.09-0.16 and 0.56-0.89 m3
s-1 during the dry and wet season, respectively (or
6-10 and 37-59 cm/day if the area is taken into account). While
surface runoff increased only 2-fold in the wet season relative
to the dry season, groundwater discharge rates increased 〜6-fold.
Since groundwater can be a major driver of surface water quality,
radon can be useful in CASS monitoring and management efforts.
Keywords: Submarine groundwater discharge; Lismore, backswamp;
Coastal hydrology; Permeable sediments; Tidal pumping』
1. Introduction
2. Material and methods
2.1. Study area
2.2. Sampling and analysis
2.3. Radon mass balance
2.3.1. Correct for other radon sources
2.3.2. Estimate minimum groundwater discharge rates
2.3.3. Estimate maximum groundwater discharge rates
3. Results
3.1. Richmond River Estuary radon survey
3.2. Tuckean Broadwater time series and Tuckean Swamp survey
3.3. Rocky Mouth Creek, Sandy Creek, and Lismore time series
3.4. Rates of groundwater discharge
4. Discussion
4.1. Radon and pH relationships
4.2. The relative contribution of groundwater inputs
4.3. Drivers of groundwater discharge
4.4. Model uncertainties
5. Conclusions
Acknowledgements
References