Recharge seasonality based on stable isotopes: Nongrowing season bias altered by irrigation in Nebraska

The sustainability of groundwater resources for agricultural and domestic use is dependent on both the groundwater recharge rate and the groundwater quality. The main purpose of this study was to improve the understanding of the timing, or seasonality, of groundwater recharge through the use of stable isotopes. Based on 768 groundwater samples collected from aquifers underlying natural resources districts in Nebraska, the isotopic composition of groundwater (δ²H and δ¹⁸O) was compared with that of precipitation by (a) mapping the isotopic composition of groundwater samples and (b) mapping a seasonality index for groundwater. Results suggest that for the majority of the state, groundwater recharge has a nongrowing season signature (October–April). However, the isotopic composition of groundwater suggests that in some intensively irrigated areas, human intervention in the water cycle has shifted the recharge signature towards the growing season. In other areas, a different human intervention (diversion of Platte River water for irrigation) has likely produced an apparent but possibly misleading nongrowing season recharge signal because the Platte River water differs isotopically from local precipitation. These results highlight the need for local information even when interpreting isotopic data over larger regions. Understanding the seasonality of recharge can provide insight into the optimal times to apply fertilizer, specifically in highly conductive soils with high leaching potential. In areas with high groundwater nitrate concentrations, this information is valuable for protecting the groundwater from further degradation. Although previous studies have framed nongrowing season recharge within the context of future climate change, this study also illustrates the importance of understanding how historical human intervention in the water cycle has affected groundwater recharge seasonality and subsequent implications for groundwater recharge and quality.     

Using three-dimensional geological mapping methods to inform sustainable groundwater development in a volcanic landscape, Victoria, Australia

This study investigated the use of three-dimensional (3D) geological methods to provide better groundwater resource estimates for the Spring Hill area in central Victoria, Australia. Geological data were gathered in 3D geological software, which was utilised to derive fundamental dimensional parameters of the groundwater system in the study area. Mining industry software and hydrogeological methods were combined to give volumetric determinations of the basalt aquifer that were used to improve estimates of the groundwater resource. The methods reduce uncertainty about the physical attributes of the aquifer systems and greatly improve conceptual understanding of their behaviour. A simple numerical water-balance model was developed to refine the estimates of aquifer volume and fluxes to approximate observed water-level behaviour in the area. This enabled a much better comparison of groundwater resource use to the natural inputs and outputs for the area. A key conclusion was that the main issues for sustainable development and use in the study area are more to do with the physical aspects of the aquifer system, rather than simply the volume of water pumped. Visualisations of the area’s hydrogeology also provide improved hydrogeological understanding and communication for groundwater users and administrators.     

Genesis of greenstone‐hosted Cu–Au mineralisation at Hill 800, Mt Useful Slate Belt,eastern Victoria

A group of structurally emplaced Cambrian metavolcanic inliers, informally known as the Barkly River greenstones, in the Mt Useful Slate Belt, eastern Victoria, host several base‐ and precious‐metal anomalies. To date, the most significant anomaly is the Hill 800 prospect, where disseminated and stockwork/stringer chalcopyrite + sphalerite + galena ± native Au mineralisation have been recognised. This ore assemblage is hosted by andesitic lavas and breccias and is syngenetic to pretectonic in origin. Deformation fabrics as well as unmineralised, low‐temperature, quartz ± carbonate veins overprint the ore minerals. Spectacular gossan exposures and intense paragonite + chlorite + muscovite hydrothermal alteration zones are associated with the known extent of the mineralisation. Field relations, geochemical, petrographic, isotopic and fluid‐inclusion studies indicate that ore genesis is closely related to a volcanic‐hosted massive sulfide (VHMS) system, although certain characteristics resemble a porphyry‐Cu style of mineralisation. Comparisons of these findings with other ore systems suggest that Hill 800 is a ‘hybrid’ system transitional between a VHMS and porphyry‐Cu style of mineralisation. Moreover, the host rocks are analogous with the economically important Mt Read Volcanic Complex of western Tasmania. These findings emphasise that the Cambrian greenstones can potentially represent a new base‐ and precious‐metal mineral province of Victoria.     

Hydraulic testing using a versatile straddle packer system for improved transmissivity estimation in fractured-rock boreholes

Equipment has been developed for straddle packer testing in fractured-rock boreholes to conduct four types of tests (constant-head step tests, slug tests, constant-rate pumping tests, and recovery tests) without deflating the packers or adjusting equipment in the hole between tests. The goal is to achieve improved accuracy and precision in the determination of transmissivity (T). Water-pressure measurements are recorded using pressure transducers positioned above and below the test interval to identify connections from the test interval to the open borehole. Insights concerning the nature of test conditions are gathered with this equipment to assess errors related to deviations from assumptions inherent in the mathematical models used to determine T, including validation of the Darcian flow assumption, validation of slug test assumptions, cross-connection to the open borehole, inadequate borehole development, and dual permeability, thereby giving greater confidence in the calculated T values. When the errors indicated above are minimized, the constant-head step tests, slug tests and constant-rate pumping/recovery tests give nearly identical values. This multiple-test approach to fractured-rock studies increases confidence in test results, which is important when the goal is characterization of fracture networks for contaminant transport and fate assessment.     

Attenuation of nitrate in aquitard sediments of southern Ontario

To examine nitrate persistence, detailed geochemical profiling, using core-squeezed water and piezometer samples, was carried out at five sites in southern Ontario where groundwater is moving downward in silt-rich aquitard sediments at rates of 16 to more than 20 cm year⁻¹. Elevated levels of NO₃⁻-N (5–50 mg 1⁻¹) that occur in the shallow groundwater as a result of agricultural activity, were found to be consistently attenuated, generally to very low levels (< 0.05 mg 1⁻¹-N), at the ‘redoxcline’, the horizon marking the boundary between the surficial weathered (brown) sediments and the underlying unweathered (grey) sediments. Tritium dating suggests that groundwater at the redoxcline depths (3–5 m) was recharged between 1970 and 1980, thus the N03 depletion appears to result from biodegradation reactions since no major landuse changes have occurred during this period. The close association of the nitrate depletion zones with the redoxcline, where, in particular, sediment sulphur contents increase abruptly, and where also porewater SO₄²⁻ levels increase, suggests that the dominant attenuation reaction is autotrophic denitrification using reduced sulphur compounds present in the unweathered sediment as the electron donor. Mass balance calculations suggest that the increase in the downward rate of migration of the redoxcline, owing to added sulphur consumption from NO₃⁻ contamination, is only about 1 mm year⁻¹ at these sites. Review of the literature indicates that most silt- and clay-rich sediments have S contents in the same range, or higher, than those investigated here, thus, in most cases where aquifers are overlain by several metres or more of unweathered confining sediments, it is likely that a high degree of protection is afforded from surficial NO₃⁻ contamination.     

Blended head analyses to reduce uncertainty in packer testing in fractured-rock boreholes

Open boreholes in fractured rock often cross-connect fractures with differing hydraulic head and the head differences between these fractures cause vertical flow in the water column. This cross-connection has potential to bias transmissivity (T) values obtained from straddle packer tests. This study demonstrates how measurements of the blended head in the open-hole segments above and below the straddle-packer test interval can be used to correct packer tests for cross-connection effects. A pressure response observed in the open-hole segment above and/or below the packers isolating a test interval during a hydraulic test indicates short-circuiting of water from the injection interval through the vertically connected fracture network to the open-hole segments, resulting in the overestimation of T. A method is presented using blended head concepts to minimize this error using a trial-and-error procedure to determine the short-circuiting flow rate to account for the head conditions in the open-hole segments during each hydraulic test. Observed differences between the measured head and the calculated blended head in the open-hole segments above and below the test interval are attributed to cross-connection effects around the 1-m-long packers. The head and corrected T values determined from packer tests are used to estimate the flow in and out of the open hole at each of the intervals tested for assessing the cross-connection effects under open borehole conditions. Understanding open-hole flow dynamics gives insight about the potential for vertical cross connection of chemical constituents caused by the open hole.