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Recharge & Groundwater-Surface Water
51 | P a g e A regional scale groundwater model was developed for part of the South East of South Australia using FloPy, a Python based interface for MODFLOW 2005. The FloPy interface allowed for transparency and reproducibility in the modelling and facilitated rapid and flexible model development that could easily couple with other models or site-specific processes. A lookup table approach for modelling net recharge (gross recharge minus ET from groundwater) was established from previous research and incorporated into the transient FloPy groundwater model. The lookup table was developed from unsaturated zone modelling using WAVES and was based on climate data, soil clay content mapping and the time-varying South Australian Land Cover Layers. This allowed the complex unsaturated zone behaviour to be captured, without significantly increasing the number of model parameters.
The modelling resulted in similar calibration performance for groundwater heads as the regional model, but with a water table that better followed the land surface between the observation bores. Sensitivity of modelled groundwater heads to recharge and evapotranspiration parameters varied, depending on depth to water table and proximity to drains and other boundary conditions. Groundwater heads near GDEs were very sensitive to recharge and ET parameterisation.
How uncertain are our recharge estimates?
Russell Crosbie 1
1. CSIRO Land and Water, Glen Osmond, SA, Australia
In many areas water allocations are determined by recharge estimates, either directly or indirectly through models. For a water balance component that can’t be directly measured it is imperative that the uncertainty in recharge estimates is quantified. It is often recommended that multiple methods should be used as a measure of uncertainty in the recharge, but this will only produce multiple recharge estimates. Different methods of estimating recharge can be estimating different quantities of water. This can be a challenge if relying on different estimation methods to provide uncertainty but can also be an opportunity for constraining recharge
estimates if recharge is estimated probabilistically. Baseflow in streams is an estimate of groundwater discharge and must be equal to or less than the groundwater recharge. The chloride mass balance gives an estimate of the net recharge (includes ET from groundwater) and must be equal to or greater than baseflow and also equal to or less than gross recharge (water that reaches the water table). The water table fluctuation method is estimating gross recharge and must be equal to or greater than the net recharge but also equal to or less than the excess water, which is calculated from remotely sensed ET subtracted from rainfall (excess water also contains the runoff component). If each of these water components can be estimated probabilistically then they can be jointly constrained using a rejection sampling approach. Using examples from recent projects we can show that
unconstrained modelled recharge estimates are highly uncertain, the range can extend over two orders of magnitude. The chloride mass balance can be estimated probabilistically with a range over a factor of 3. By jointly constraining multiple recharge estimates the uncertainty can be reduced closer to a factor of 2.
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Estimating recharge from recirculated groundwater with dissolved gases: an end-member mixing analysis
Shawan Dogramaci 1 , Peter Cook 2 1. RioTinto Iron Ore, Perth, WA, Australia
2. National Centre for Groundwater Research and Training (NCGRT), College of Science and Engineering, Flinders University, Bedford Park, SA, Australia
Groundwater circulation is an important process in irrigation, mining activities and town water supplies. Tracers that undergo different degrees of re-equilibration with the atmosphere during this recirculation process can enable ambient groundwater and recirculated groundwater to be differentiated. In this paper, the recirculated groundwater has been pumped to dewater open pit mines and discharged into ephemeral creeks. Some of this water subsequently recharged back into the aquifer.
CFC-12 (which completely re-equilibrates on exposure to the atmosphere), 14C (partial re-equilibration) and 3H (no re-equilibration) are used in a four end-member mixing analysis to differentiate between (1) ambient groundwater, (2) recirculated groundwater, (3) river recharge from natural flows prior to commencement of mining operations (in 2007), and (4) natural river recharge post-2007. Sampling of the surface water when discharge of mine water was the only source of river flow enabled the extent of re-equilibration of both CFC-12 and 14C to be accurately determined. Since CFC-12 re-equilibrates more rapidly than 14C, recirculating groundwater had a CFC-12 concentration which was close to modern, but a 14C activity that was higher than the original groundwater, but less than modern recharge. Since 3H does not re-equilibrate with the atmosphere, it enabled
differentiation between stream recharge due to infiltration of mine water discharge and that due to natural creek flows. Uncertainty of end-member compositions is due to changes in the end-member concentrations over time in the case of natural river flows, uncertainty in the extent of tracer re-equilibration for the groundwater
recirculation end-member, and spatial variations in the composition of the ambient groundwater end-member. The mean uncertainty of end-member fractions was estimated to be less than 12%.
Evaluation of natural and anthropogenic factors on changes in river discharge and groundwater exchange in a Mekong sub-basin
Somphasith Douangsavanh 1 , Okke Batelaan 1 , Eddie Banks 1 , Pavelic Paul 2 1. Flinders University, Bedford Park, SA, Australia
2. International Water Management Institute, IWMI, Vientiane, Laos
Besides natural climate variability exacerbated by induced climate change, regulation of large surface water reservoirs and connected downstream irrigation systems have well-documented consequences on river discharge patterns. However, these natural and anthropogenic changes may also affect groundwater fluxes, which needs
understanding for sustainable water resources planning and management. In the Nam Ngum River Basin, Laos, a major tributary of the Mekong River, hydropower dams were developed in the upper part of the basin, whilst the lower part is mostly undammed and intensively developed for agriculture and domestic water supply. This study investigates the changes to the river flow pattern and groundwater storage from the pristine period (pre-dam) to the near–present period (post-dam). Firstly, we analyzed observed discharge data to detect historical changes in river discharge in comparison with historical climate trend analyses. Secondly, we analyzed how
53 | P a g e natural and anthropogenic changes influenced groundwater storage in the sub-basin by looking at the incremental discharge between two gauging stations compared to total Equivalent Water Heights derived from the Gravity Recovery and Climate Experiment (GRACE). The results show that: (i) during the pristine conditions, the river discharge was highly seasonally dynamic. However, under post-dam conditions, there is diminished seasonal response with increased dry season flows and decreased monsoonal peak flows. This phenomenon showed a greater impact of human-made water infrastructures on river discharge than dependence on variability of climatic factors, as no trends in wet and dry season precipitation (Mann-Kendall test) are found in the times series; (ii) Incremental discharge increased by 38% compared to the pristine period, which is understood to be a result of higher groundwater
discharge due to an increasing trend in total water storage as indicated by GRACE.
Future research is underway to deconvolve the factors that are driving the changes in river and groundwater exchange.
Increased groundwater and contaminant discharge to surface water in response to catchment loading
Tamie R. Weaver 1 , Gavin Powell 2 , Dana Windle 1 , Dora Kovacsy 1 1. ERM Australia, Melbourne, VIC, Australia
2. ERM Australia, Newcastle, NSW, Australia
Introduction: Groundwater discharge to surface water can transfer contamination from groundwater to streams. This paper describes conditions where the ratio of groundwater contributing to surface water flows increased with increased rainfall and catchment wetting. This is not consistent with the generally accepted model of
groundwater discharge dominating streamflow in dry periods; rather, it highlights the dynamics of the groundwater discharge process.
Methodology: Groundwater discharge processes were assessed at sites where groundwater and surface water chemistry allowed their relative inputs to be evaluated. Initial “spot” monitoring events indicated that, at times, surface water quality became more similar to groundwater even when surface water flows had increased. Sites that were instrumented with level, and in some locations EC, loggers in groundwater and streams provided increased understanding of groundwater
discharge dynamics.
Results: At one site, spring discharge became more consistent with groundwater than surface water in periods of higher rainfall, indicating that hydraulic loading of the catchment increased groundwater discharge to the surface. At an intermittent flowing stream instrumented with EC and level loggers, after stream flows had been established for several weeks, later increases in surface water salinity continued to occur. This was considered to be related to further wetting of the catchment
increasing hydraulic gradients and, consequently, groundwater discharge, even with higher stream flow rates. At a third site, during a temperate winter, surface water salinity continued to increase in response to increasing groundwater discharge until sufficient rainfall events had occurred to reverse hydraulic gradients between the stream and groundwater.
Summary and conclusions: The results highlight the importance of temporal monitoring of stream and groundwater systems to assess the dynamics of groundwater discharge to surface water. Level and EC logger data provide a framework in which to assess temporal variability of groundwater discharge under different surface flow conditions.
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Quantifying air−water gas exchange in rivers and lakes using high- resolution time series of dissolved atmospheric gases
Peter G Cook 1 , Ulrich W Weber 2 , Ma
tthias S Brennwald 3 , Rolf Kipfer 3 , Thomas C Stieglitz 4 1. NCGRT, Flinders University, Adelaide, SA, Australia
2. Department of Geosciences, University of Oslo, Oslo, Norway
3. Department of Water Resources and Drinking Water, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf , Switzerland
4. Aix-Marseille Université, CNRS, IRD, INRA, Coll France, CEREGE, Aix-en-Provence, France Gas exchange across the air-water interface is a key parameter when using gas tracers such as radon to quantify groundwater discharge to surface waters. We present a novel method for quantifying the gas transfer velocity based on recently developed techniques for the in situ, near continuous measurement of dissolved gases with a field portable mass spectrometer. Concentrations of gases in surface water show diurnal variations due to diurnal changes in water temperature (and thus gas solubility). However, variation in observed dissolved gas concentrations are damped and lagged with respect to equilibrium concentrations, the extent of which depends upon the diurnal temperature variation, the water depth, and the gas transfer velocity. The method fits a model to the measured gas concentrations to derive the gas transfer velocity from the amplitude and the phase lag between observed and equilibrium concentrations. With the current experimental setup, the method is sensitive to gas transfer velocities of 0.05 – 9 m/day (for N2), at a water depth of 1 m, and a given daily water temperature variation of 10 ºC. Experiments were performed (a) in a controlled experiment to prove the concept and to confirm the capability to determine low transfer velocities, and (b) in a field study in a shallow coastal lagoon covering a range of transfer velocities.
Assessment of PFAS as a novel tool for estimating groundwater recharge and aquifer characteristics
John M. Bradd 1
1. GHD, Sydney, NSW, Australia
Optimum sustainable groundwater management requires the collection of detailed hydrogeological parameters. One of the most important parameters is the recharge rate, which can be challenging to determine by conventional methods. Various methods have been utilised to study groundwater recharge mechanisms for several decades including environmental and artificial tracer techniques. In more recent years, contaminated sites investigations requiring the analysis of Per- and Poly Fluoroalkyl Substances (PFAS) in groundwaters have led to a large number of data sets available in aquifer systems. This provides an opportunity to assess its value in estimating recent recharge and recharge rates that can be important input values for modelling and managing the groundwater system.
The premise for determine recharge rates from PFAS concentration is that PFAS can be considered a conservative tracer in groundwater flow in a similar way that
chloride or tritium travels through groundwater systems conservatively. Since the time that PFAS enters the groundwater system is known from historical records, the maximum depth and distance that PFAS travels in the groundwater system can
55 | P a g e provide a good estimate of the travel time and therefore provides a recharge rate for that system.
Publicly available data sets were utilised in this preliminary assessment of PFAS as a novel approach to recharge estimation. Investigation sites with water quality data that included PFAS measurements and key hydrogeological data such as bore lithology, SWL, multiple screen-depths, and other aquifer properties were collected and analysed. The assessment utilised data from sites that had the most detailed hydrogeological information and examined minimum data requirements.
The results from this initial assessment indicate that providing that a site
investigation has a well-planned and detailed monitoring network, PFAS can be used to quantify recharge rates in a groundwater system. Given that there are many water quality/contamination studies across a range of aquifer systems requiring the measurement of PFAS, this data can be utilised not only for determining if a site exceeds health and environmental criteria, but can be used to provide valuable model input data and sustainable groundwater management for regulators in the future.