•δ34SSO4 and δ18OSO4 were successfully used to identify the origin of SO4.•Stable isotope signatures are particularly useful for identification pyrite oxidation.•Classification for identification of ...the acid rock drainage has been developed.
Sulphur (S) is a commonly occurring element in most aquifers, primarily in oxidised (sulphates) and reduced (sulphides) forms. Sulphides often constitute a risk to groundwater quality due to acid rock drainage, especially in catchments that are subject to mining excavations or groundwater injection. However, in semi-arid regions detection of the acid rock drainage risk can be challenging and traditional methods based on observations of increasing SO4 concentrations or SO4/Cl ratios in surface and groundwater, are not necessarily applicable. In addition, decreasing pH, usually accompanying pyrite oxidation, can be masked by the high pH-neutralisation capacity of carbonate and silicate minerals.
Analysis of 73 surface and groundwater samples from different water bodies and aquifers located in the Hamersley Basin, Western Australia found that most of the samples are characterised by neutral pH but there was also a large spatial variability in the dissolved sulphate (SO4) concentrations that ranged from 1mg/L to 15,000mg/L. Not surprisingly, groundwater in aquifers that contained pyrite had high sulphate concentrations (>1000mg/L). This was associated with low δ34SSO4 values (+1.2‰ to +4.6‰) and was consistent with the values obtained from aquifer matrix pyritic rock samples (−1.9‰ to +4.4‰). It was also found that the SO4 concentrations and acidity levels were not only dependent on δ34SSO4 values and existence of pyrite but also on the presence of carbonate minerals in the aquifer matrix. The groundwater in aquifers containing both pyrite and carbonate minerals had a neutral pH and was also saturated with respect to gypsum and had high magnesium concentrations of up to 2200mg/L suggesting de-dolomitisation as the process buffering the acidity generated by pyrite oxidation.
Based on the findings from this study, a classification scheme has been developed for identification of the acid rock drainage contribution to groundwater that encompasses a myriad of geochemical processes that occur in aqueous systems. The classification uses five proxies (SO4, SO4/Cl, SI of calcite, δ34SSO4 and δ18OSO4) to improve assessment of the oxidation of sulphide potential contribution to overall sulphate ion concentrations regardless of acidity levels of the aqueous system.
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•Quantification of evaporation is essential for calibration of hydrological models.•We designed a new software Hydrocalculator based on the updated Craig–Gordon model.•Hydrocalculator ...estimates evaporative losses from changes in water isotope composition.•The software was verified using field pan experiments and published data.•Estimation of isotope composition of air vapours is the major source of uncertainty.
Accurate quantification of evaporative losses to the atmosphere from surface water bodies is essential for calibration and validation of hydrological models, particularly in remote arid and semi-arid regions, where intermittent rivers are generally minimally gauged. Analyses of the stable hydrogen and oxygen isotope composition of water can be used to estimate evaporative losses from individual pools in such regions in the absence of instrumental data but calculations can be complex, especially in highly variable systems. In this study, we reviewed and combined the most recent equations required for estimation of evaporative losses based on the revised Craig–Gordon model. The updated procedure is presented step-by-step, increasing ease of replication of all calculations. The main constraints and sources of uncertainties in the model were also evaluated. Based on this procedure we have designed a new software, Hydrocalculator, that allows quick and robust estimation of evaporative losses based on isotopic composition of water. The software was validated against measures of field pan evaporation under arid conditions in northwest Australia as well as published data from other regions. We found that the major factor contributing to the overall uncertainty in evaporative loss calculations using this method is uncertainty in estimation of the isotope composition of ambient air moisture.
► Isotope composition of rain is governed by evaporation of droplets and rainout effect. ► Only infrequent large-volume rainfalls in the wet season recharge aquifers. ► Hydrochemistry of groundwater ...is controlled by evaporation and carbonate equilibrium. ► Most of the recharge occurs through alluvium sediments.
The Hamersley Basin, in the semi-arid Pilbara region of northwest Australia, is currently subject to increasing pressure from altered hydrology associated with mining activities as well as water abstraction for regional development. Sustainable water management across the region must be underpinned by an understanding of the factors that constrain water supply in arid zones. We measured the amount and isotopic signature of individual rainfall events over three consecutive years (2009–2011) to determine the likely processes that control surface water pools in streams and groundwater recharge across the Hamersley Basin. We also measured concentrations of ions (in particular bromide and chloride) to define and quantify sources of major recharge. Stable isotope composition of precipitation across the basin forms a Local Meteoric Water Line (LMWL) defined by the equation: δ2H=7.03±0.17×δ18O+4.78±1.45. Thus, the slope of the LMWL was similar to the Global Meteoric Water Line (GMWL). However, the intercept of the LMWL was significantly different to the GMWL, which is attributable to the amount or “rainout” effect. The stable isotope composition of rainfall events was highly variable and dependent on event size. However, the δ2H and δ18O values of fresh groundwater from the alluvium and fractured aquifers were similar and characterised by a very narrow range (alluvium aquifer δ18O −8.02±0.83‰, δ2H −55.6±6.0‰, n=65; fractured aquifer δ18O −8.22±0.70‰, δ2H −56.9±5.0‰, n=207). Our findings suggest that intense rainfall events of >20mm with limited evaporation prior to infiltration contribute most to recharge. In contrast, the δ2H and δ18O values and chemical composition of the relatively saline groundwater in the terminal Fortescue Marsh suggest a combination of evaporation and cyclic drying and wetting of the marsh surface prior to recharge. Saline groundwater samples were more 18O enriched than fresh groundwater; δ2H and δ18O values shifted to the right of the LMWL, forming a straight line with a slope of 3.58±0.20 and an intercept of −25.55±0.71 (R2=0.95, p<0.001, n=18). The stable isotope mass balance for most of the surface water pools in the basin show significant evaporation and are highly enriched compared to underlying groundwater. We conclude that significant seepage from highly evaporated pools to groundwater is very limited.
•An ephemeral creek in the semi-arid Australia received mine discharge for>6years.•Numerical modelling, Cl and isotope mass balance were used to assess impact.•Cl increased from 73 to 120mg/L and ...δ18O from −8.2‰ to −7.0‰ along the creek.•Water loss by recharge accounts for ∼65% and evapotranspiration by ∼35%.•Combined multi-method approach allowed successful calculation of water budget.
Dewatering associated with mining below water table to achieve dry mining conditions may exert significant pressure on water balance in terms of lowering the water table and change in the dynamics of interactions between surface water and groundwater. The discharge of surplus mine water into ephemeral streams may also affect the water balance, by elevating groundwater levels and altering the exchange rate between streams and underlying aquifers. However, it is unclear whether volumes and recharge processes are within the range of natural variability. Here, we present a case study of an ephemeral creek in the semi-arid Hamersley Basin of northwest Australia that has received continuous mine discharge for more than six years. We used a numerical model coupled with repeated measurements of water levels, chloride concentrations and the hydrogen and oxygen stable isotope composition (δ2H and δ18O) to estimate longitudinal evapotranspiration and recharge rates along a 27km length of Weeli Wolli Creek. We found that chloride increased from 73 to 120mg/L across this length, while δ18O increased from −8.2‰ to −7.00‰. Groundwater is directly connected to the creek for the first 13km and recharge rates are negligible. Below this point, the creek flows over a highly permeable aquifer and water loss by recharge increases to a maximum rate of 4.4mm/d, which accounts for ∼65% of the total water discharged to the creek. Evapotranspiration losses account for the remaining ∼35%. The calculated recharge from continuous flow due to surplus water discharge is similar to that measured for rainfall-driven flood events along the creek. Groundwater under the disconnected section of the creek is characterised by a much lower Cl concentration and more depleted δ18O value than mining discharge water but is similar to flood water generated by large episodic rainfall events. Our results suggest that the impact of recharge from continuous flow on the water balance of the creek has not extended beyond 27km from the discharge point. Our approach using a combination of hydrochemical and isotope methods coupled with classical surface flow hydraulic modelling allowed evaluation of components of the water budget otherwise not possible in a highly dynamic system that is mainly driven by infrequent but large episodic floods.
Hydraulic head and groundwater age data are effective in building understanding of groundwater systems. Yet their joint role in detecting and characterising low-permeability geological structures, ...i.e. hydrogeologic barriers such as faults and dykes, has not been widely studied. Here, numerical flow and transport models, using MODFLOW-NWT and MT3D-USGS, were developed with different hydrogeologic barrier configurations in a hypothetical aquifer. Computed hydraulic head and groundwater age distributions were compared to those without a barrier. The conjoint use of these datasets helps in detecting vertically-oriented barriers. Two forms of recharge were compared: (1) applied across the entire aquifer surface (uniform), and (2) applied to the upstream part of the aquifer (upgradient). The hydraulic head distribution is significantly impacted by a barrier that penetrates the aquifer’s full vertical thickness. This barrier also perturbs the groundwater age distribution when upgradient recharge prevails; however, with uniform recharge, groundwater age is not successful in detecting the barrier. When a barrier is buried, such as by younger sediment, hydraulic head data also do not clearly identify the barrier. Groundwater age data could, on the other hand, prove to be useful if sampled at depth-specific intervals. These results are important for the detection and characterisation of hydrogeologic barriers, which may play a significant role in the compartmentalisation of groundwater flow, spring dynamics, and drawdown and recovery associated with groundwater extraction.
•Measuring in-well flow regimes improves key hydrogeological data and insight.•Efficient new field method and numerical model for a single-well tracer test.•Pumped flow profile quantified by ...inverting tracer dilution profiles.•Steady-state approach is suited to a wide range of pumping flow rates.•Analysis of mixed convection helps assess and avoid fluid/tracer density issues.
Quantifying vertical flows in long-screened or open wells is essential for their reliable use in all types of groundwater investigations. In ambient conditions, a flow profile shows the producing/receiving zones of head-driven flow, the relative vertical head gradient in the aquifer system and potential bias in chemistry samples. A flow profile while pumping can be used to quantify aquifer heterogeneity and the sampled water mixtures. This paper describes a novel approach to a single-well tracer test to quantify the flow regime in a pumped well, which is unique in its utility over a wide range of discharge rates. During constant pumping, a tracer is injected at the opposite end of the well and, as it is drawn towards the pump, the tracer is diluted in proportion to each inflow. A dilution model using the advection-dispersion equation is used to visually fit a flow profile that explains all tracer profiles (pre-injection, transient phase and steady-state). Results compare favourably to borehole EM flowmeter data, particularly if tracer density issues are correctly interpreted and head-loss in the flowmeter is avoided. A dimensionless Froude number is provided to assist both with understanding and minimising the role of free convection when planning all types of in-well tracer tests involving a density contrast. Like the flowmeter, this method is particularly suited to screened wells, where packers are ineffective. Used together or separately in existing wells, these in-well methods can provide considerable information on aquifer-well hydraulics without the cost of additional drilling.
The study site, the Mount Bruce Flats, is located in the Pilbara, a semiarid region of northern Western Australia.
Semiarid climates are characterised by an extreme water deficit, with evaporation ...exceeding precipitation several-fold. Groundwater recharge is episodical and occurs only after infrequent but very large volume precipitation, while all other precipitation events are quickly lost to evaporation. We investigated the contribution of an internally drained basin to localised groundwater recharge by combining the results of calculations from various tracer methods. We developed a new mobile-immobile recharge transport model coupled with a probability model based on Markov Chain Monte Carlo simulations that combines several age and hydrochemical tracers to estimate localised infiltration.
The actual recharge varied greatly across the basin, being 4–5 times higher in the central part compared to the margins. The highest probability for recharge was calculated for 2.5 mm×y−1 and 12.6 mm×y−1, with a median value of 6.8 mm×y−1 while the median mobile effective porosity was low at ∼0.5 %. The flood plains overlying freshwater aquifers may contribute several times more to localised recharge than the upper parts of the catchments, which are characterised by fast runoff and a lower water retention time. Therefore, these endorheic basins could be a valuable source of water for groundwater recharge or constitute an additional hydrological challenge for mining.
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•The multitracer approach is essential for robust evaluation of recharge.•Highest probability for recharge was at 2.5 mm×y−1 and 12.6 mm×y−1.•The median mobile effective porosity was relatively low at 0.5 %.•The chloride mass balance underestimate recharge estimating 0.5–8 mm×y−1.•Endorheic basins are critically important for replenishing groundwater.
Streams in hot, arid environments often exist as a series of isolated pools along main channels. During these periods, shallow alluvial through flow may strongly influence key ecological processes ...within pools. We measured diel changes in
δ
13
C values of dissolved inorganic carbon (DIC) and dissolved oxygen (DO) in two pools of ephemeral, dryland streams. We quantified alluvial water connectivity through stable isotope analysis (
δ
18
O and
δ
2
H) of pool and alluvial water. We also estimated gross primary productivity (GPP) and ecosystem respiration (ER) rates across a wider set of pools in both streams.
δ
13
C
DIC
values displayed regular diel cycles, where both pools displayed small but similar daily amplitude (0.7–0.9‰) despite contrasting amplitudes of change in DO (0.8 mg L
−1
vs. 2.8 mg L
−1
) and contrasting alluvial water connectivity (connected vs. disconnected). Water temperature was the strongest predictor of both
δ
13
C
DIC
values and rates of change in
δ
13
C
DIC
across both pools. Across both streams, all pools were net heterotrophic. GPP (0.35–1.73 g O
2
m
−2
d
−1
) and ER (0.49–2.64 g O
2
m
−2
d
−1
) rates were linked to aquatic vegetation cover. The disconnect between diurnal amplitudes of
δ
13
C values and DO concentrations thus suggests that ecological drivers of gas exchange became increasingly localised as pools contracted.
Strontium and carbon isotopes are used to study the effect of carbonate mineral dissolution, and inter-aquifer mixing on the chemical evolution of groundwater from the Murray Group Limestone Aquifer ...in semi-arid SE Australia. The
87Sr/
86Sr ratio of groundwater evolves from 0.7097 at the basin margin to a less radiogenic value of 0.7084 about 250
km down-gradient, which is similar to the
87Sr/
86Sr ratio of the aquifer carbonate matrix. The concomitant increase in
δ
13C
DIC, Sr/Ca and Mg/Ca ratios in the groundwater along a 250
km transect suggests that incongruent dissolution of high Mg-calcite controls the carbonate geochemistry in this aquifer. Further down-gradient, the groundwater is characterized by a relatively more radiogenic
87Sr/
86Sr ratio caused by upward leakage and mixing with more radiogenic groundwater from the Renmark Group Sand Aquifer. A mixing model using
87Sr/
86Sr ratio suggests that the fraction of water that contribute to the Murray Group Aquifer through upward leakage from underlying Renmark Group Aquifer ranges from 15 to 85%.
The Precambrian meta-sedimentary fractured rock aquifers of the Hamersley Basin in northwest Australia are some of the oldest water-bearing formations on the planet and host enormous iron ore ...deposits. Groundwater is the only permanent source of water in the basin, therefore understanding the hydrological processes that effect water quality and quantity is a pre-requisite for sustainable water management. We used a combination of major dissolved ion concentrations, including Sr and Ca, in combination with δ2H, δ18O and δ87Sr in flood water and groundwater as tracers to constrain the processes affecting groundwater chemistry. The δ87Sr composition of groundwater in three major aquifer types ranges from 11.8‰ to 40.6‰ and reflects the mineralogy of altered Precambrian dolomite (15.1‰ to 55.4‰) rather than the host iron ore formations (22.5‰ to 46.5‰ >95% iron oxides) or highly radiogenic shale bands and clay minerals (200‰ to 2322.5‰). Groundwater in the terminal Fortescue Marsh wetland of the basin has a rather constant δ87Sr signature of 36.6±1.4‰ irrespective of variations in TDS, δ18O and Sr concentration. This groundwater is considered to be mature in a geochemical sense, representing the final stage of water evolution on a basin scale. Mixing calculations utilising δ87Sr and Ca/Sr data demonstrate contributions of salts from three major sources: on average >92% from precipitation, ~7% from carbonate rocks and <1% from rocks with highly radiogenic signatures (shales and clays). These results demonstrate groundwater evolution from a recharge area to discharge area at the regional scale, but more importantly that water quality in the terminal wetland is primarily driven by rainfall chemistry in floodwaters rather than water–rock interactions in the catchment.
•δ87Sr — clays and shale was 115 to 2322‰, mafic volcanic rocks and altered carbonates −0.5 to 8.2‰ and dolomite −5.9‰.•The ultimate δ87Sr signature of water in the Hamersley Basin, one of the oldest landscapes (2.7Ga), equals 36.6±1.4‰.•Waters with this signature can be considered as “mature” in a geochemical sense.•>92% of solutes originate from precipitation, ~7% from volcanic and carbonate rocks and <1% shale and clay minerals.•Vertical flow is the dominant mode compared to lateral groundwater flow.
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