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Constraining water fluxes through the streambed of a semi-arid losing stream using natural tracers: heat and radioisotopes
(American Geophysical Union, 2011-12-05) Andersen, MS; Rau, GC; McCallum, AM; Meredith, KT; Acworth, RI
Natural physical and chemical tracers of flow have different advantages and shortfalls based on their properties and the uncertainty related to variability in their source concentration. Each tracer integrates over a characteristic spatial-temporal scale depending on its decay or production rate and the flow velocity of the system. For instance heat tracing using diurnal temperature fluctuations will, at best, provide information about flow in the upper 1-2 m of the streambed before the signal is dampened below measurement resolution (Constantz et al. 2003). Conversely, radioisotopes used as tracers will integrate over increasing spatio-temporal scales for decreasing decay constants. Radioisotopes with comparatively slow decay rates will be less sensitive for resolving flow conditions on short spatio-temporal scales. Therefore, it is difficult to use these tracers in the streambed of losing systems because the radioactive decay is not discernible against the variability. Consequently, employing a combination of different tracers provides information on different parts of a given flow system. Comparing flow velocities derived from tracers integrating over different scales allows for separating the local hyporheic exchange from the regional groundwater recharge. A field experiment was carried out in a perennial section of the mostly ephemeral Maules Creek in NSW, Australia. Streambed temperature profiles were monitored at three sites along a 400 m stretch of the perennial reach. Streambed temperatures were recorded at 4 depths within one meter below the streambed. Water samples were collected from surface water, streambed and groundwater and analysed for stable water isotopes (18O and 2H) and radioisotopes (222Rn and 3H). The streambed heat profiles provided time series of surface water/groundwater exchange. Using this method it was found that the conditions were losing at all three sites with recharge rates varying between 0 and 0.4 m/d. 222Rn measurements in the surface water along the perennial reach qualitatively identified losing and gaining sections of the stream with low and high 222Rn activities, respectively. One of the losing sections of the stream was instrumented with a transect of groundwater piezometers. In this transect, 3H levels of 1.3-1.5 TU were measured, comparable to surface waters, indicating recent groundwater recharge. However, the variations in 3H combined with the analysis uncertainty did not allow for a recharge estimate. 222Rn with its half-life of only 3.8 d proved more useful. A zone of low 222Rn activity was found as deep as 6-7 m below the stream, corroborating the 3H and temperature data. Regional groundwater 222Rn activities were used to estimate the secular equilibrium activity of Rn. Residence times of 1 to 7 days were calculated based on these estimates. Converted to Darcy velocities of 0.2-1.7 m/d these values generally agree with the velocities derived from the temperature data indicating that the measured fluxes from the temperature data represent recharge rates and not simply hyporheic exchange. © American Geophysical Union.
Investigation of surface-water/groundwater interactions using environmental isotopes (2H, 18O, 14C and 3H) in the Maules Creek Catchment, NSW, Australia
(Australasian Environmental Isotope Conference, 2009-12-03) Andersen, MS; McCallum, AM; Meredith, KT; Acworth, RI
The objectives of this study were to assess surface water/groundwater interactions and to estimate recharge for the Maules Creek Catchment (NSW), a sub-catchment of the Murray-Darling Basin. Surface water and groundwater were sampled for environmental isotopes 2H, 18O, 14C and 3H. Within the catchment groundwater abstraction, mainly for the irrigation of cotton, has been carried out since the mid 1980s. An average decline of groundwater levels of about 4 to 5 m has been observed over the last 30 years [1]. Flow in the Namoi River also appears to have become more intermittent over the same period [2]. The stable water isotope data from the catchment shows that there is a distinct contrast between the regional groundwater and the river water, with the river water having an evaporative signature [3]. Shallow groundwater (<20 m) in proximity of the river (0.1-1 km) generally shows a mixed stable isotope signature indicating river water recharging the aquifer and mixing with the regional groundwater. Although this data is useful in identifying end-member sources, it does not provide an indication of the groundwater residence time or rate of river recharge. The replenishment rates of the aquifer were investigated using 3H and 14C data. The uncorrected 14C results generally indicate increasing apparent groundwater ages with depth beneath the ground surface, with the deepest water having apparent radiocarbon ages of up to 21,000 yrs (Fig. 1). However, noticeable differences to this pattern are observed. Near the Namoi River, older groundwater is generally found at much shallower depths than anticipated (Fig. 1 – black squares). This indicates an upward discharge of groundwater into the river (gaining river conditions). It is uncertain whether this is a relict of past discharge patterns because presently there appears to be little or no discharge of groundwater to the river. Other variations in the 14C results observed with depth occur in some areas near the river, where groundwater abstraction is causing large seasonal drawdown, here the opposite pattern is observed with modern water found at depths of up to 60 m (Fig. 1 – white squares). It appears that the origin of this modern groundwater is recently infiltrated river water (losing river conditions) entering the aquifer due to the lowered groundwater levels. The diffuse (rain-fed) recharge to the aquifer has been estimated in this study by ignoring the samples close to the river which are considered to be either recharge or discharge zones. A simple exponential age-depth relationship (insert in Fig. 1) was obtained by assuming a homogeneous isotropic box shaped aquifer with uniform depth, recharge and porosity. Based on this, a long term diffuse recharge of 6 mm/yr was estimated. This is an initial first estimate of recharge conditions for the system and is subject to changes in the age distribution caused by deviations from the assumptions. The scatter observed in the data shows that the aquifer is most likely not homogeneous and isotropic. Further validation and assessment of these recharge rates will be presented based on 3H and corrected 14C results to verify this model. This study shows that the changes in the surface water/groundwater interactions, as implied by the isotopes, impacts on the catchment water balance and especially on the fluxes entering the river from the aquifer. The preliminary interpretation of the data suggests quite low recharge rates which will have implications for sustainable groundwater management in this part of the Murray-Darling Basin.
Investigation of δ18O and δ2H in the Namoi River catchment - elucidating recharge sources and the extent of sur-face water/groundwater interaction
(The Authors, 2008-10-26) Andersen, MS; Meredith, KT; Timms, W; Acworth, RI
Stable isotopes 18O and 2H were analysed in water samples from rainfall, surface water and groundwater within the semi-arid Namoi River catchment in NSW, Australia.The isotopic composition of rainfall events and groundwater samples plot along the Local Meteoric Water Line (LMWL). In contrast, the surface water samples of the Namoi River clearly show signs of evaporative enrichment and plot on a Local Evaporation Line (LEL) constructed for the area based on δ18O and δ2H time-series for surface waters of the Namoi River. The river samples have a distinctly lower slope than the LMWL which is due to evaporation. Shallow groundwater near the Namoi River shows considerable enrichment compared to average groundwater signatures and plots in between the LMWL and the LEL on a δ2H vs. δ18O graph. These results clearly indicate that the Namoi River is recharging the shallow aquifer system. Conversely, the isotopic composition of surface water in the tributaries of Maules and Horsearm creeks are similar to groundwater indicating that these creeks are receiving groundwater discharge. This study reveals many complex hydrological processes occurring in the catchment. It would not have been possible to elucidate these processes without the use of stable isotope data. © Authors
Investigations of the impact on surface water/groundwater interactions using carbon-14
(Australian Institute of Nuclear Science and Engineering (AINSE), 2009-11-25) McCallum, AM; Andersen, MS; Meredith, KT; Acworth, RI
Groundwater resources are increasingly being developed across the globe, particularly in arid to semi-arid regions where, in periods of drought, groundwater is heavily relied upon. Over abstraction of groundwater has the potential to impact surface water flows in these catchments. However, the processes and timescales at which this impact operates are generally poorly understood. Groundwater dating using carbon-14 can aid in the understanding of long-term surface water/groundwater interaction processes. © 2009 AINSE
A radioisotope tracer study of estuarine goundwater movement of the Eastern Australian coast
(International Atomic Energy Agency, 2003-05-19) Hughes, CE; Stone, DJM; Acworth, RI
Groundwater discharge to coastal waters, and the associated transport of nutrients and contaminants, is believed to have a significant impact on coastal ecosystems. However, complex boundary conditions and rapidly changing short-term fluxes make net flows resulting from local or regional groundwater discharge difficult to quantify. Such boundary conditions include beach face wave runup and storm setup, sub-surface tidal forcing, frequent surface inundation and tidally driven surface/groundwater interactions in estuarine and coastal areas. Tracer techniques can complement hydrological and geochemical studies of such systems and help distinguish longer-term net fluxes from the highly variable short-term fluxes. At Hat Head, NSW, on the eastern Australian coast, a comprehensive study of hydrogeology and hydrogeochemistry has being conducted in an estuarine/coastal sand dune aquifer. A scheme currently under construction will dispose of treated sewage effluent from the small coastal community by injection into the sand dune aquifer. Geochemical and isotopic data from the site indicate a high degree of complexity showing evidence of regional groundwater flow occurring at depth and more localised and highly dynamic conditions in the top 10 m of the aquifer. Tritium data indicate that regional groundwater is modern and stable isotope ratios have been used to distinguish between salt flat and sand dune dominated systems where evaporative and seawater mixing processes are observed. Storm wave setup and beach wave runup have been shown to elevate the water table near the coast leading to flow reversal and potential discharge of effluent to the estuarine zone. A radioisotope tracer study of groundwater flow in response to tidal forcing was conducted adjacent to a tidal creek at Hat Head. Using the short-lived radioisotope conservative tracer, bromine-82, groundwater movement was tracked in-situ over a period of ∼5 days on two occasions encompassing both neap and spring tide conditions. The tracer was injected into a screened borehole and gross gamma counts monitored from an adjacent borehole using a variety of collimated NaI detectors as well as down-hole gamma spectrometry. This technique maps the path of the slow moving tracer without sampling and allows the net groundwater movement to be distinguished from short term tidally driven fluxes. Results from the environmental isotope and radiotracer studies will be presented. © The authors.
Use of stable and radioactive isotopes to unravel surface water groundwater interactions in developed catchment
(International Atomic Energy Agency, 2015-01-01) Andersen, MS; Rau, GC; Meredith, KT; McCallum, AM; Acworth, RI
Abstract: Traditionally surface water groundwater interactions are estimated using hydrometric and water balance methods. However, the hydrometric methods, based on hydraulic gradients, have huge uncertainties related to the mostly unknown and often highly heterogeneous permeability distribution. Similarly, the surface water groundwater exchange can be relatively small compared to other components of the water balance (e.g. differential steam flow gauging) and consequently its estimation is associated with large uncertainties when using a water balance method. Environmental tracers such as isotopes on the other hand integrate what has actually taken place. However, each individual isotopic tracer has particular advantages and disadvantages and therefore limited use. For instance each radioactive isotope is limited to provide residence time estimations within a certain age range whereas stable isotopes only work when different water sources have distinctly different isotopic compositions. In this study, of a highly developed catchment subject to groundwater depletion it is demonstrated that by combining different tracers (14C, 3H, 2H and 18O) a more complete picture of the surface water groundwater interactions can be obtained.

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