Sébastien Lamontagne

Quantifying groundwater discharge to Cockburn River, southeastern Australia, using dissolved gas tracers 222Rn and SF6

Groundwater discharge to the Cockburn River, southeast Australia, has been estimated from comparison of natural 222Rn activities in groundwater and river water, interpreted using a numerical flow model that simulates longitudinal radon activities as a function of groundwater inflow, hyporheic exchange, evaporation, gas exchange with the atmosphere, and radioactive decay. An injection of SF6 into the river to estimate the gas transfer velocity assisted in constraining the model. Previous estimates of groundwater inflow using 222Rn activities have not considered possible input of radon due to exchange between river water and water in the hyporheic zone beneath the streambed. In this paper, radon input due to hyporheic exchange is estimated from measurements of radon production by hyporheic zone sediments and rates of water exchange between the river and the hyporheic zone. Total groundwater inflow to the Cockburn River is estimated to be 18500 m 3/d, although failure to consider hyporheic exchange would cause overestimation of the volume of groundwater inflow by approximately 70%. Copyright 2006 by the American Geophysical Union.

Groundwater use by riparian vegetation in the wet–dry tropics of northern Australia

Within Australia and globally there is considerable concern about the potential impacts of groundwater extraction on ecosystems dependent on groundwater. In this study we have combined heat pulse and isotopic techniques to assess groundwater use by riparian vegetation along the Daly River in the Northern Territory. The riparian forests of the Daly River exhibited considerable structural and floristic complexity. More than 40 tree species were recorded during vegetation surveys and these exhibited a range of leaf phonologies, implying complex patterns of water resource partitioning within the riparian forests. Water use was a function of species and season, and stand water use varied between 1.8 and 4.1u2009mm day–1. In general, however, water use tended to be higher in the wet season than during the dry season, reflecting the contribution to stand water use by dry-season deciduous tree species. There was a strong relationship between stand basal area and stand water use in the wet season, but the strength of this relationship was lower in the dry season. The amount of groundwater use, as determined by analysis of deuterium concentrations in xylem sap, was principally a function of position in the landscape. Trees at lower elevations, closer to the river, used more groundwater than trees higher on the levees. By using a combination of techniques we showed that riparian vegetation along the Daly River was highly groundwater dependent and that these water-use requirements need to be considered in regional management plans for groundwater.

Sulfidic materials in dryland river wetlands

Due to a combination of river regulation, dryland salinity and irrigation return, lower River Murray floodplains (Australia) and associated wetlands are undergoing salinisation. It was hypothesised that salinisation would provide suitable conditions for the accumulation of sulfidic materials (soils and sediments enriched in sulfides, such as pyrite) in these wetlands. A survey of nine floodplain wetlands representing a salinity gradient from fresh to hypersaline determined that surface sediment sulfide concentrations varied from <0.05% to ~1%. Saline and permanently flooded wetlands tended to have greater sulfide concentrations than freshwater ones or those with more regular wetting–drying regimes. The acidification risk associated with the sulfidic materials was evaluated using field peroxide oxidations tests and laboratory measurements of net acid generation potential. Although sulfide concentration was elevated in many wetlands, the acidification risk was low because of elevated carbonate concentration (up to 30% as CaCO3) in the sediments. One exception was Bottle Bend Lagoon (New South Wales), which had acidified during a draw-down event in 2002 and was found to have both actual and potential acid sulfate soils at the time of the survey (2003). Potential acid sulfate soils also occurred locally in the hypersaline Loveday Disposal Basin. The other environmental risks associated with sulfidic materials could not be reliably evaluated because no guideline exists to assess them. These include the deoxygenation risk following sediment resuspension and the generation of foul odours during drying events. The remediation of wetland salinity in the Murray–Darling Basin will require that the risks associated with disturbing sulfidic materials during management actions be evaluated.

Measurements of riverbed hydraulic conductivity in a semi-arid lowland river system (Murray–Darling Basin, Australia)

Riverbed hydraulic conductivity (Kr) was measured along one river reach in four tributaries of the Murray–Darling Basin (MDB) in south-eastern Australia. Two techniques were trialled: in-river falling-head tests in high Kr sediments, and laboratory evaporation tests on intact riverbed cores for low Kr sediments. In-river falling-head tests were conducted using two types of permeameter: a steel-base permeameter or a stand-pipe permeameter. Kr was found to range from 10–10 to 10–3u2009mu2009s–1, corresponding to a range in riverbed sediment textures from clay to silty gravels, respectively. Although the within-reach variability in Kr was also large, in general the river reaches could be divided in two groups, those with a low Kr ( 10–5u2009mu2009s–1). The low Kr reach (Billabong Creek) was a clay-lined bed, whereas the others had silty sand or silty gravel beds. Thus, regional-scale assessments of Kr in the MDB could be made using a stratified sampling process in which reaches would be first classified into low or high Kr classes, and then Kr measurements made in a subsample of low and high Kr reaches. This would be an improvement over the current practice whereby riverbed Kr is estimated either from regional soil maps or through the calibration of groundwater models.

Ground truthing groundwater-recharge estimates derived from remotely sensed evapotranspiration: a case in South Australia

Using a water balance to estimate groundwater recharge through the use of remotely sensed evapotranspiration offers a spatial and temporal density of data that other techniques cannot match. However, the estimates are uncertain and therefore ground truthing of the recharge estimates is necessary. This study, conducted in the south-east of South Australia, demonstrated that the raw water-balance estimates of recharge had a negative bias of 45xa0mm/yr when compared to 190 recharge estimates using the water-table fluctuation method over a 10-year period (2001–2010). As this bias was not related to the magnitude of the recharge estimated using the water-table fluctuation method, a simple offset was used to bias-correct the water-balance recharge estimates. The bias-corrected recharge estimates had a mean residual that was not significantly different from an independent set of 99 historical recharge estimates but did have a large mean absolute residual indicating a lack of precision. The value in this technique is the density of the data (250-m grid over 29,000xa0km2). The relationship between the water-table depth and net recharge under different vegetation types was investigated. Under pastures, there was no relationship with water-table depth, as the shallow roots do not intercept groundwater. However, under plantation forestry, there was a relationship between net recharge and water-table depth. Net recharge under plantation forestry growing on sandy soils was independent of the water table at around 6xa0m depth but, under heavier textured soils, the trees were using groundwater from depths of more than 20xa0m.RésuméL’utilisation de bilan d’eau pour estimer la recharge des eaux souterraines via le recours à la mesure de l’évapotranspiration par télédétection fournit une densité spatiale et temporelle de données que d’autres techniques ne peuvent égaler. Toutefois, les estimations de recharge sont incertaines et par suite leur confirmation au sol est nécessaire. Cette étude, menée dans le Sud-Est du Sud de l’Australie, montre que le bilan d’eau estimé présente un biais négatif de 45xa0mm/an sur 190 estimations de recharge basées sur la méthode de fluctuation de la nappe sur une période de 10 ans (2001–2010). Comme l’incidence de ce biais n’a pas été rapportée à l’amplitude de la recharge estimée en utilisant la méthode de fluctuation de la surface de la nappe libre, une simple correction a été faite pour affranchir la recharge du biais. Ces estimations de la recharge corrigée présentent une moyenne résiduelle lissée ne différent pas de manière significative d’un ensemble indépendant de 99 estimations de recharges historiques mais les moyennes absolues résiduelles présentent un large écart indiquant un manque de précision. L’intérêt de cette technique est la densité des données (grille de 250xa0m sur 29,000xa0km2). La relation entre la profondeur de la nappe et la recharge nette pour différents types de végétation a été étudiée. Sous les pâturages, il n’y a pas de relation avec la profondeur de la nappe, car les racines de faible profondeur n’interceptent pas l’eau souterraine. Cependant, sous couvert forestier, il existe une relation entre la recharge nette et la profondeur de la nappe. La recharge nette sous les plantations forestières sur sol sableux est indépendante du niveau piézométrique situé à environ 6xa0m de profondeur, mais dans des sols plus texturés les arbres prélèvent l’eau souterraine à des profondeurs supérieures à 20xa0m.ResumenEl uso de un balance de agua para estimar la recarga de agua subterránea a partir de la evapotranspiración obtenida por teledetección ofrece una densidad espacial y temporal de datos que otras técnicas no pueden brindar. Sin embargo, las estimaciones son inciertas y por lo tanto es necesaria una verificación de campo de la estimación de la recarga. Este estudio, realizado en el sudeste de Australia del Sur, demostró que la estimación del balance bruto de agua tuvo un sesgo negativo de 45xa0mm/año cuando se la comparó con la estimación de 190 de recarga calculada usando el método de las fluctuaciones del nivel freático en un período de 10 años (2001–2010). Como este sesgo no estaba relacionado con la magnitud de la recarga estimada de acuerdo al método de la fluctuación del nivel freático, se usó una compensación simple para corregir el sesgo de la estimación de recarga del balance de agua. La estimación de la recarga con el sesgo corregido tuvo una media residual que no fue significativamente distinta a conjunto independiente de 99 estimaciones históricas de recarga pero tenían un gran residuo absoluto medio indicando una falta de precisión. El valor en esta técnica es la densidad de los datos (una grilla de 250-m en 29,000xa0km2). Se investigó la relación entre la profundidad del nivel freático y la recarga neta bajo diferentes tipos de vegetación. Bajo pastura no había relación con la profundidad del nivel freático, dado por las raíces poco profundas que no interceptan el agua subterránea. Sin embargo, bajo plantaciones forestales, había una relación entre la recarga neta y la profundidad del nivel freático. La recarga neta bajo plantaciones forestales en crecimiento en suelos arenosos fue independiente del nivel freático hasta alrededor de 6xa0m de profundidad, pero en suelos de texturas más pesadas los árboles usan agua subterránea a profundidades de más que 20xa0m.摘要通过轻微感知的蒸发蒸腾量利用水平衡估算地下水补给量提供了其它技术不能匹配的数据时空密度。然而,估算数不确定,因此,需要脚踏实地的补给估算数。这项在南澳大利亚东南部进行的研究证明,原水平衡补给估算数与采用水位波动方法获得的10年期(2001–2010年)190个补给估算数相比有45 mm/yr的负偏差。因为这个偏差与采用水位波动方法估算的补给幅度无关,因此,采用简单的补偿校正了水平衡补给估算数的偏差。这个偏差校正过的补给估算数有平均残差,平均残差与99个历史的补给估算数独立集没有显著差别,但具有很大的平均绝对残差,表明缺乏精度。这项技术的价值就是数据的密度(29,000 km2内250-m的 网格)。研究了不同植被类型条件下水位深度和净补给量的关系。在牧场之下,与水位深度没有关系,因为浅根并不拦截地下水。然而,在人工林之下,净 补给量和水纹深度有关系。在生长在砂质土壤上的人工林之下,净补给量独立于大约6米深的水位之外,但在较重结构土壤下,树木汲取深度超过20 米的地下水。ResumoA estimativa da recarga da água subterrânea a partir de um balanço hidrológico calculado com os dados de evapotranspiração detetada remotamente permite definir uma densidade espacial e temporal dos dados que outras técnicas não conseguem relacionar. No entanto, as estimativas são incertas, e por isso é necessário autenticar a recarga no terreno. O presente estudo, realizado no sudeste da Austrália Meridional, demonstrou que a estimativa da recarga calculada através do balanço hídrico da água total apresenta um viés negativo de 45xa0mm/ano quando comparado com os 190xa0mm/ano de recarga estimados a partir do método de flutuação do nível da água aplicado a um período de 10 anos (2001–2010). Como o viés calculado não estava relacionado com a magnitude da recarga estimada pelo método de flutuação do nível da água, foi usado um método simples para corrigir o viés da estimativa da recarga a partir do balanço hidrológico. O viés da recarga estimada corrigida apresentou uma média residual semelhante à recarga estimada a partir de um conjunto de 99 dados históricos independentes, apresentando no entanto uma média residual absoluta elevada, indicativa de falta de precisão. O valor desta técnica está na densidade dos dados (malha de 250xa0m numa área total de 29,000xa0km2). Foi investigada a relação entre a profundidade do nível freático e a recarga total em áreas cobertas por diferentes tipos de vegetação. Em terrenos de pastagem não se observou qualquer relação com o nível freático, uma vez que as raízes pouco profundas não intercetam a água subterrânea. Já em terrenos ocupados por espécies arbóreas, foi possível estabelecer uma relação entre a recarga total e a profundidade do nível freático. Observou-se também que, nas plantações florestais em solos arenosos, a recarga total é independente do nível freático a partir de cerca de 6xa0m de profundidade, mas, em solos com texturas mais pesadas, as árvores usam a água subterrânea a partir de profundidades superiores a 20xa0m.

Impact of a drought on nutrient concentrations in the Lower Lakes (Murray Darling Basin, Australia)

Abstract Nutrient concentrations increased in 2 lakes (Lake Alexandrina and Lake Albert) located at the downstream end of the Murray Darling Basin, Australia, as a result of water level drawdown and salinisation associated with a severe drought. Between January 2007 and March 2008 the salinity difference between the inlet and outlets (5 barrages) increased from 0.9 to 21.0 g L−1, resulting primarily from seawater leakage through the barrages. Subsequently, in relatively sheltered areas upstream of the barrages, permanent salinity-derived density stratification developed, leading to the development of an anoxic hypolimnion. This seemingly favoured the regeneration of dissolved nutrients from the sediments, with standing stocks of ammonium and filterable reactive phosphorus increasing by 250 and 142%, respectively. However, the source of leakage water through the barrages also contributed to the increase. While dissolved organic carbon concentrations also increased, this was a result of evapoconcentration because calculated standing stocks changed little during the study period. In the open water areas, vertical density stratification was not evident, but sediment resuspension seemed to increase during the drawdown. Total organic nitrogen and total phosphorus concentrations were closely related to light attenuation, suggesting increased resuspension of particulate nutrients during the water level drawdown or increased assimilation of dissolved nutrients by phytoplankton. Overall, sediment resuspension seemed to have had a greater impact on nutrient concentrations in open water areas of Lake Alexandrina and Lake Albert, while saline intrusions were more significant in relatively sheltered areas located close to the lake outlets.

Submarine groundwater discharge from the South Australian Limestone Coast region estimated using radium and salinity

The Tertiary Limestone Aquifer (TLA) is one of the major regional hydrogeological systems of southern Australia. Submarine groundwater discharge (SGD) of freshwater from the TLA occurs through spring creeks, beach springs and diffusively through beach sands, but the magnitude of the total flux is not known. Here, a range of potential environmental tracers (including temperature, salinity, (222)Rn, (223)Ra, (224)Ra, (226)Ra, (228)Ra, and (4)He) were measured in potential sources of SGD and in seawater along a 45xa0km transect off the coastline to evaluate SGD from the TLA. Whilst most tracers had a distinct signature in the sources of water to the coastline, salinity and the radium quartet had the most distinct SGD signal in seawater. A one-dimensional advection-dispersion model was used to estimate the terrestrial freshwater component of SGD (Qfw) using salinity and the recirculated seawater component (Qrsw) using radium activity in seawater. Qfw was estimated at 1.2-4.6xa0m(3)xa0s(-1), similar in magnitude to previously measured spring creek discharge (∼3xa0m(3)xa0s(-1)) for the area. This suggests that other terrestrial groundwater discharge processes (beach springs and diffuse discharge through beach sands) were no more than 50% of spring creek discharge. The largest component of total SGD was Qrsw, estimated at 500-1000xa0m(3)xa0s(-1) and possibly greater. The potential for wave, storm, or buoyancy-driven porewater displacement from the seafloor could explain the large recirculation flux for this section of the Southern Ocean Continental Shelf.

Groundwater delivery rate of nitrate and predicted change in nitrate concentration in Blue Lake, South Australia

Blue Lake, the principal water supply for the City of Mount Gambier (South Australia), is contaminated with nitrate (NO3–) from polluted groundwater. Using existing data, a study was undertaken to determine the past load of NO3– from groundwater entering the lake and to forecast future trends in lake NO3– concentration. Groundwater NO3– loads for the 1971–1997 period were estimated with an inverse model, which combined the long-term record for NO3– concentration in the lake with a simple NO3– mass-balance. Model results show that the load of NO3– from groundwater (18–24 metric tons (t) year–1 as N) was by far the largest source to Blue Lake between 1971 and 1997. Sinks for NO3– included pumping withdrawal (10–14 t year–1), in-lake consumption (7–10 t year–1), and groundwater outflow (0–1.8 t year–1). The NO3– concentration in incoming groundwater (4–7 mg N L–1) appears to have increased slowly but steadily during the 1971–1997 period (at a rate varying between 0.037 and 0.070 mg N L–1 year–1). By assuming that the rate of increase in groundwater NO3– concentration will remain constant, a forecast for lakewater NO3– concentration was made for the 1998–2028 period. Lakewater NO3– concentration should increase from the contemporary ~3.5 mg N L–1 to 4 or 5 mg N L–1 by 2028. In the short term (decades), the rate of pumping withdrawal will be the main determinant of NO3– concentration in the lake through its impact on the rate of groundwater inflow and the lake water residence time. Although the drinking water guideline for NO3– (11. 3 mg N L–1) may not be exceeded in the short term (decades), it may be exceeded in the longer term (centuries) as NO3– concentration in the neighbouring aquifer adjusts to the contemporary land use.