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Xie Y.,Flinders University | Cook P.G.,Flinders University | Cook P.G.,Water for a Healthy Country National Research Flagship | Brunner P.,University of Neuchatel | And 2 more authors.
Groundwater | Year: 2014

Decline in regional water tables (RWT) can cause losing streams to disconnect from underlying aquifers. When this occurs, an inverted water table (IWT) will develop beneath the stream, and an unsaturated zone will be present between the IWT and the RWT. The IWT marks the base of the saturated zone beneath the stream. Although a few prior studies have suggested the likelihood of an IWT without a clogging layer, most of them have assumed that a low-permeability streambed is required to reduce infiltration from surface water to groundwater, and that the IWT only occurs at the bottom of the low-permeability layer. In this study, we use numerical simulations to show that the development of an IWT beneath an unclogged stream is theoretically possible under steady-state conditions. For a stream width of 1 m above a homogeneous and isotropic sand aquifer with a 47 m deep RWT (measured in an observation point 20 m away from the center of the stream), an IWT will occur provided that the stream depth is less than a critical value of 4.1 m. This critical stream depth is the maximum water depth in the stream to maintain the occurrence of an IWT. The critical stream depth decreases with stream width. For a stream width of 6 m, the critical stream depth is only 1 mm. Thus while theoretically possible, an IWT is unlikely to occur at steady state without a clogging layer, unless a stream is very narrow or shallow and the RWT is very deep. © 2014. Source

Shanafield M.,Flinders University | Cook P.G.,Flinders University | Cook P.G.,Water for a Healthy Country National Research Flagship | McCallum J.,Flinders University | Simmons C.T.,Flinders University
Water Resources Research | Year: 2012

Existing analytical solutions to determine aquifer response to a change in stream stage are inappropriate where an unsaturated zone exists beneath the stream, as in the case of disconnected stream-aquifer systems. A better understanding of the relationship between aquifer response and transient stream stage in disconnected systems is therefore required, as this would also aid in the field determination of the status of connection between the stream and aquifer. We use a numerical model to examine transient stream stage and the corresponding water table response. Beneath disconnected streams, the magnitude of head change in the water table level is a balance between the cumulative infiltration during a flow event and the rate at which the water can disperse laterally. Increases in wave duration, stream width, and streambed permeability result in greater infiltrated water volume and therefore a higher peak response at the water table. Conversely, higher aquifer transmissivity and aquifer hydraulic conductivity allow the water to move laterally away from the stream faster, resulting in a smaller head change below the stream. Lower unsaturated storage results in a greater and faster aquifer response because the unsaturated zone can fill more quickly. Under some combinations of parameters, the magnitude of the disconnected head response is more than seven times greater than the change in stream stage driving streambed infiltration; an effect which can never occur beneath a connected stream. The results of this sensitivity analysis are compared to field data from a river in eastern Australia to determine periods of disconnection. Where the change in aquifer head is greater than the change in stream stage, disconnection between the stream and aquifer can be determined. © 2012 American Geophysical Union. All Rights Reserved. Source

Batlle-Aguilar J.,Flinders University | Cook P.G.,Flinders University | Cook P.G.,Water for a Healthy Country National Research Flagship
Water Resources Research | Year: 2012

An infiltration experiment at the stream reach scale was performed to estimate infiltration rates beneath an ephemeral, losing stream during streamflow events. At a time when the stream was dry, a 7 m stream section was dammed upstream and downstream using metal sheets. During a 5 day period water was pumped into the isolated section of the stream, and the surface water level was maintained at three successive increasing stages. The infiltration rate at each water level was thereby equal to the pumping rate required to maintain that water level. The advantages of the method are that it samples a much greater area than traditional methods and provides information on infiltration through stream banks as well as through the streambed. Experimental results provide insight into transient infiltration and recharge processes beneath ephemeral streams. Although the experiment continued for 5 days, infiltration during the first hour accounted for 19% of the total infiltration. High transient infiltration rates were also observed following each increase in stream stage. Experimental infiltration rates were used to calibrate a two-dimensional model developed within Hydrus, which was subsequently used to estimate infiltration associated with a natural flow event in the same stream reach. During the natural flow event, the total infiltration was 33% greater than would have been estimated assuming steady state infiltration rates. Dry antecedent moisture content controls the transient infiltration rate and hence increases the total infiltrated volume during flow events, but it does not increase the aquifer recharge. © 2012. American Geophysical Union. Source

Cook P.G.,Flinders University | Cook P.G.,Water for a Healthy Country National Research Flagship
Hydrological Processes | Year: 2013

Environmental tracer methods have been used to quantify groundwater discharge to rivers for the past few decades. A number of different tracers have been used in these studies, including individual ion concentrations, electrical conductivity, stable isotopes 2H and 18O, and the dissolved gases helium, chlorofluorocarbons and radon. This paper discusses the assumptions of the method, as well as its resolution and accuracy. The method will be most accurate when the tracer concentration in groundwater is very distinct from that in the river. On the basis of typical parameters, groundwater inflow rates as low as 5mm/day can usually be estimated with electrical conductivity and ion tracers. A lower limit of resolution of approximately 2mm/day is usually possible with radon, principally because the ratio of the river concentration to the groundwater concentration will be higher. However, hyporheic exchange can also contribute radon to the river. Where this process is significant, it is more difficult to estimate groundwater inflow from radon activities in the river, thus reducing the accuracy of the method. For CFCs, the lower limit of resolution is approximately 30mm/day. Helium has not been widely used but can potentially be very accurate if the groundwater is old. The method assumes steady-state conditions and so can only be applied when river flows are stable. Sampling resolution is also particularly important for dissolved gases, and uncertainty in where groundwater inflow occurs between sampling points can cause large uncertainty in inflow rates if the distance between sample locations is large. Poor mixing of solutes within the river can limit the method if the river is wide and shallow. When correctly applied, however, the environmental tracer method is able to provide robust estimates of groundwater discharge at a scale and accuracy that is not possible with most other methods. Copyright © 2012 John Wiley & Sons, Ltd. Source

Page D.,CSIRO | Dillon P.,CSIRO | Toze S.,CSIRO | Toze S.,Water for a Healthy Country National Research Flagship | And 4 more authors.
Water Research | Year: 2010

A quantitative microbial risk assessment (QMRA) was performed at four managed aquifer recharge (MAR) sites (Australia, South Africa, Belgium, Mexico) where reclaimed wastewater and stormwater is recycled via aquifers for drinking water supplies, using the same risk-based approach that is used for public water supplies. For each of the sites, the aquifer treatment barrier was assessed for its log10 removal capacity much like for other water treatment technologies. This information was then integrated into a broader risk assessment to determine the human health burden from the four MAR sites. For the Australian and South African cases, managing the aquifer treatment barrier was found to be critical for the schemes to have low risk. For the Belgian case study, the large treatment trains both in terms of pre- and post-aquifer recharge ensures that the risk is always low. In the Mexico case study, the risk was high due to the lack of pre-treatment and the low residence times of the recharge water in the aquifer. A further sensitivity analysis demonstrated that human health risk can be managed if aquifers are integrated into a treatment train to attenuate pathogens. However, reduction in human health disease burden (as measured in disability adjusted life years, DALYs) varied depending upon the number of pathogens in the recharge source water. The beta-Poisson dose response curve used for translating rotavirus and Cryptosporidium numbers into DALYs coupled with their slow environmental decay rates means poor quality injectant leads to aquifers having reduced value to reduce DALYs. For these systems, like the Mexican case study, longer residence times are required to meet their DALYs guideline for drinking water. Nevertheless the results showed that the risks from pathogens can still be reduced and recharging via an aquifer is safer than discharging directly into surface water bodies. © 2009 Elsevier Ltd. Source

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