James L. McCallum

An Analysis of River Bank Slope and Unsaturated Flow Effects on Bank Storage

Recognizing the underlying mechanisms of bank storage and return flow is important for understanding streamflow hydrographs. Analytical models have been widely used to estimate the impacts of bank storage, but are often based on assumptions of conditions that are rarely found in the field, such as vertical river banks and saturated flow. Numerical simulations of bank storage and return flow in river-aquifer cross sections with vertical and sloping banks were undertaken using a fully-coupled, surface-subsurface flow model. Sloping river banks were found to increase the bank infiltration rates by 98% and storage volume by 40% for a bank slope of 3.4° from horizontal, and for a slope of 8.5°, delay bank return flow by more than four times compared with vertical river banks and saturated flow. The results suggested that conventional analytical approximations cannot adequately be used to quantify bank storage when bank slope is less than 60° from horizontal. Additionally, in the unconfined aquifers modeled, the analytical solutions did not accurately model bank storage and return flow even in rivers with vertical banks due to a violation of the dupuit assumption. Bank storage and return flow were also modeled for more realistic cross sections and river hydrograph from the Fitzroy River, Western Australia, to indicate the importance of accurately modeling sloping river banks at a field scale. Following a single wet season flood event of 12 m, results showed that it may take over 3.5 years for 50% of the bank storage volume to return to the river.

Bias of apparent tracer ages in heterogeneous environments.

The interpretation of apparent ages often assumes that a water sample is composed of a single age. In heterogeneous aquifers, apparent ages estimated with environmental tracer methods do not reflect mean water ages because of the mixing of waters from many flow paths with different ages. This is due to nonlinear variations in atmospheric concentrations of the tracer with time resulting in biases of mixed concentrations used to determine apparent ages. The bias of these methods is rarely reported and has not been systematically evaluated in heterogeneous settings. We simulate residence time distributions (RTDs) and environmental tracers CFCs, SF6 , (85) Kr, and (39) Ar in synthetic heterogeneous confined aquifers and compare apparent ages to mean ages. Heterogeneity was simulated as both K-field variance (σ(2) ) and structure. We demonstrate that an increase in heterogeneity (increase in σ(2) or structure) results in an increase in the width of the RTD. In low heterogeneity cases, widths were generally on the order of 10 years and biases generally less than 10%. In high heterogeneity cases, widths can reach 100 s of years and biases can reach up to 100%. In cases where the temporal variations of atmospheric concentration of individual tracers vary, different patterns of bias are observed for the same mean age. We show that CFC-12 and CFC-113 ages may be used to correct for the mean age if analytical errors are small.

Episodic recharge and climate change in the Murray-Darling Basin, Australia

In semi-arid areas, episodic recharge can form a significant part of overall recharge, dependant upon infrequent rainfall events. With climate change projections suggesting changes in future rainfall magnitude and intensity, groundwater recharge in semi-arid areas is likely to be affected disproportionately by climate change. This study sought to investigate projected changes in episodic recharge in arid areas of the Murray-Darling Basin, Australia, using three global warming scenarios from 15 different global climate models (GCMs) for a 2030 climate. Two metrics were used to investigate episodic recharge: at the annual scale the coefficient of variation was used, and at the daily scale the proportion of recharge in the highest 1% of daily recharge. The metrics were proportional to each other but were inconclusive as to whether episodic recharge was to increase or decrease in this environment; this is not a surprising result considering the spread in recharge projections from the 45 scenarios. The results showed that the change in the low probability of exceedance rainfall events was a better predictor of the change in total recharge than the change in total rainfall, which has implications for the selection of GCMs used in impact studies and the way GCM results are downscaled.RésuméDans les régions semi arides la recharge épisodique peut constituer une partie importante de la recharge totale dépendant d’épisodes pluvieux rares. Avec des prévisions de changement climatique suggérant de futurs changements d’ampleur et d’intensité des précipitations, la recharge de nappe dans les régions semi arides sera vraisemblablement affectée de façon disproportionnée par le changement climatique. Cette étude cherchait à examiner les changements liés à une recharge épisodique dans les zones arides du Murray-Darling Basin, Australie, en utilisant trois scénarios de réchauffement global tirés de 15 modèles différents (GCMs) du climat 2030. Deux grandeurs ont été utilisées pour examiner la recharge épisodique: le coefficient de variation à l’échelle de l’année, et le pourcentage de recharge au plus haut 1% du jour à l’échelle journalière. Les grandeurs étaient proportionnelles mais ne permettaient pas de conclure si la recharge épisodique devait ou non augmenter dans cet environnement; ceci n’est pas un résultat surprenant étant donnée la dispersion des projections de recharge des 45 scénarios. Les résultats ont montré que le changement dans la probabilité basse d’un excédent des événements pluvieux était un meilleur indicateur du changement de recharge totale que le changement de précipitation totale, ce qui a des implications dans la sélection des GCM utilisés dans les études d’impact et la façon dont les résultats du GCM sont réduits.ResumenEn áreas semiáridas la recarga episódica puede formar una parte significativa de la recarga total, dependiendo de los eventos infrecuentes de las precipitaciones. Con las proyecciones de cambio climático sugiriendo cambios en las intensidades y magnitudes de las futuras precipitaciones, probablemente la recarga del agua subterránea en área semiáridas se verá afectada desproporcionadamente por el cambio climático. Este estudio buscó investigar los cambios proyectados en la recarga episódica en áreas áridas de la cuenca Murray-Darling, Australia, usando tres escenarios de calentamiento global a partir de 15 diferentes modelos climáticos globales (GCMs) para el clima en 2030. Se utilizaron dos métricas para investigar la recarga episódica: en escala anual se usó el coeficiente de variación, y en la escala diaria la proporción de la recara en un 1% más alto de la recarga diaria. Estas métricas eran recíprocamente proporcionales pero no fueron concluyentes acerca si la recarga episódica iba a incrementarse o disminuir en este ambiente; esto no es un resultado sorprendente considerando la amplitud en las proyecciones de la recarga a partir de 45 escenarios. Los resultados mostraron que el cambio en la baja probabilidad de eventos de excedencia de las precipitaciones fue un mejor predictor del cambio en la recarga total, lo cual tiene implicancias para la selección de GCMs usados en estudios de impacto y la manera en que los resultados del GCM son llevados a escalas más reducidas.摘要在半干旱地区,间歇性补给量能占到总补给量的很大一部分,取决于不频发的降雨事件。由于气候变化会导致未来降雨大小和强度的变化,半干旱地区的地下水补给很可能不成比例地受气候变化影响。本研究根据从15个不同的全球气候模型(GCMs)中选择的3个全球暖化情景,分析了2030年的气候条件,调查了澳大利亚Murray-Darling盆地干旱地区地下水间歇性补给的变化。利用两个变量来调查间歇性补给:年尺度上的变量系数以及日尺度上日补给量最高1%的补给比例。变量二者之间是成比例的,但是这个环境条件下间歇性补给量到底是增加还是降低的,则是不确定的,考虑到45个情景中补给变化的广度,这个结果并不奇怪。结果表明超过的降雨事件的低概率变化相比总的降雨变化,是更好地总补给量变化的预测方法,这对影响研究中全球气候模型的选择以及该模型结果的尺度缩小方法是非常有意义的.ResumoA recarga episódica em zonas semi-áridas pode constituir uma parte significativa da recarga total, dependente de eventos de precipitação raros. Com as projeções para as alterações climáticas a sugerir futuras modificações na grandeza e na intensidade da precipitação, é provável que a recarga da água subterrânea em zonas semi-áridas venha a ser afectada desproporcionadamente pelas alterações climáticas. Este estudo procurou investigar as alterações projectadas da recarga episódica em zonas áridas da Bacia de Murray-Darling, Austrália, usando três cenários de aquecimento global em 15 diferentes modelos climáticos globais (GCM) para um clima em 2030. Foram usadas duas métricas para investigar a recarga episódica: à escala anual foi usado o coeficiente de variação e à escala diária usou-se a proporção de recarga das maiores percentagens de recarga diária. As métricas foram proporcionais entre si, mas foram inconclusivas quanto à possibilidade da recarga episódica ir aumentar ou diminuir neste ambiente, o que não é um resultado surpreendente, considerando a dispersão das projeções de recarga a partir dos 45 cenários. Os resultados mostraram que as mudanças na baixa probabilidade de superação de eventos de precipitação foi um melhor preditor das modificações na recarga total, do que a mudança na precipitação total, o que tem implicações para a selecção dos GCM utilizados em estudos de impacte e na forma como os resultados de GMC são aplicados localmente.

Limitations of the Use of Environmental Tracers to Infer Groundwater Age

Apparent ages obtained from the measured concentrations of environmental tracers have the potential to inform recharge rates, flow rates, and assist in the calibration of groundwater models. A number of studies have investigated sources of error in the relationships between the apparent ages, and the age assumed by models to relate this quantity to an aquifer property (e.g., recharge). These studies have also provided a number of techniques for correcting the known biases of apparent ages. In this paper, we review some of the concepts of age bias. We then demonstrate this bias through the use on four numerical examples, and assess the accuracy of correction methods in overcoming this bias. We examine this for CFCs, SF6, 3H/3He, 39Ar, and 14C. We demonstrate that in our four simple steady-state aquifer examples, bias occurs for a wide range of environmental tracers and flow configurations. When applying correction methods, we found that the values obtained are limited by the model assumptions. Models accounting for exchange with aquitards represent whole mobile zones and not discrete well screens. Mean transit times (comparable to mean ages) obtained from lumped parameter models deviate from actual values as the assumed distribution varies from the actual distribution. Methods that use multiple tracer ages are limited to ranges where both tracers report apparent ages. Our findings suggest that the incorporation of environmental tracer data into the understanding of groundwater systems requires approaches such as the direct use of concentrations, or the simulation of full age distributions.

Nonparametric estimation of groundwater residence time distributions: What can environmental tracer data tell us about groundwater residence time?

Residence time distributions (RTDs) have been used extensively for quantifying flow and transport in subsurface hydrology. In geochemical approaches, environmental tracer concentrations are used in conjunction with simple lumped parameter models (LPMs). Conversely, numerical simulation techniques require large amounts of parameterization and estimated RTDs are certainly limited by associated uncertainties. In this study, we apply a nonparametric deconvolution approach to estimate RTDs using environmental tracer concentrations. The model is based only on the assumption that flow is steady enough that the observed concentrations are well approximated by linear superposition of the input concentrations with the RTD; that is, the convolution integral holds. Even with large amounts of environmental tracer concentration data, the entire shape of an RTD remains highly nonunique. However, accurate estimates of mean ages and in some cases prediction of young portions of the RTD may be possible. The most useful type of data was found to be the use of a time series of tritium. This was due to the sharp variations in atmospheric concentrations and a short half-life. Conversely, the use of CFC compounds with smoothly varying atmospheric concentrations was more prone to nonuniqueness. This work highlights the benefits and limitations of using environmental tracer data to estimate whole RTDs with either LPMs or through numerical simulation. However, the ability of the nonparametric approach developed here to correct for mixing biases in mean ages appears promising.

Residence times of stream‐groundwater exchanges due to transient stream stage fluctuations

The biogeochemical functioning of stream ecosystems is heavily dependent on water and water-borne nutrient fluxes between the stream itself and the streambed and banks (i.e., the hyporheic zone). The travel time of water exchanges through the hyporheic zone has been investigated previously; however, these studies have primarily modeled exchanges under steady state conditions assuming spatial pressure variations. This assumes that the hydraulic gradients that drive the exchanges are maintained the whole time the stream water remains in the bed or banks, which is unrealistic. Therefore, in this study we use a transient approach to investigate residence time distributions (RTDs) of bank inflow and bank outflow during both regular, diurnal stream stage variations and storm flow events. We demonstrate that RTDs reflect the timing and magnitude bank inflows, rather than smooth RTDs. We also show that small percentages of water from a given bank inflow event may be present in bank outflows for long periods of time, due to dispersion and diffusion within the bank, and lower rates of bank outflow, relative to bank inflow. This is apparent in the synthetic model of a single storm flow event, where 10% remained in the bank after 50 days. Additionally, residence times for a given bank inflow event are longer when repeated events occur, because the bank outflows from one event are “interrupted” by an increase in stream stage during a successive event. For example, field data capturing events of variable timing and magnitude showed that 70 days after each of three storm flow events occurred, 40, 12 and 30% of the bank inflow event remained in the banks. These cases indicate that bank exchanges are temporally dynamic and the RTDs of return flows can have significant tailing, which will dictate rates of nutrient exchange within the near-stream environment.

Residence times of groundwater and nitrate transport in coastal aquifer systems: Daweijia area, northeastern China

Groundwater within the coastal aquifer systems of the Daweijia area in northeastern China is characterized by a large of variations (33-521mg/L) in NO3(-) concentrations. Elevated nitrate concentrations, in addition to seawater intrusion in the Daweijia well field, both attributable to anthropogenic activities, may impact future water-management practices. Chemical and stable isotopic (δ(18)O, δ(2)H) analysis, (3)H and CFCs methods were applied to provide a better understanding of the relationship between the distribution of groundwater mean residence time (MRT) and nitrate transport, and to identify sources of nitrate concentrations in the complex coastal aquifer systems. There is a relatively narrow range of isotopic composition (ranging from -8.5 to -7.0‰) in most groundwater. Generally higher tritium contents observed in the wet season relative to the dry season may result from rapid groundwater circulation in response to the rainfall through the preferential flow paths. In the well field, the relatively increased nitrate concentrations of groundwater, accompanied by the higher tritium contents in the wet season, indicate the nitrate pollution can be attributed to domestic wastes. The binary exponential and piston-flow mixing model (BEP) yielded feasible age distributions based on the conceptual model. The good inverse relationship between groundwater MRTs (92-467years) and the NO3(-) concentrations in the shallow Quaternary aquifers indicates that elevated nitrate concentrations are attributable to more recent recharge for shallow groundwater. However, there is no significant relationship between the MRTs (8-411years) and the NO3(-) concentrations existing in the carbonate aquifer system, due to the complex hydrogeological conditions, groundwater age distributions and the range of contaminant source areas. Nitrate in the groundwater system without denitrification effects could accumulate and be transported for tens of years, through the complex carbonate aquifer matrix and the successive inputs of nitrogen from various sources.

Identifying modern and historic recharge events from tracer-derived groundwater age distributions

Understanding groundwater ages offers insight into the time scales of recharge, aquifer storage turnover times, and contaminant protection time frames. The ability to quantify groundwater age distributions heavily depends on the choice of the interpretive model, and often important features of the age distribution cannot be identified with the subset of available models. In this paper, we implemented a multiple tracer method using a technique that assumes limited details regarding the shape of the age distribution and applied it to dewatering wells at a mine site in the Pilbara region of north-western Australia. Using our method, we were able to identify distinct age components in the groundwater. We calculated the presence of four distinct age groups in the samples. All wells contained water aged between zero and 20 years. However, the rest of the samples were composed of water between 50 and 100 years, 100 and 600 years, or water approximately 1000 years old. These were consistent with local recharge sources (50–100 years) and knowledge of paleoclimate from lake sediment records. We found that although the age components were well constrained, the relative proportions of each component were highly sensitive to errors of environmental tracer data. Our results show that our method can identify distinct age groups in groundwater samples without prior knowledge of the age distribution. The presence of distinct recharge times gives insight into groundwater flow conditions over long periods of time.

A general reactive transport modeling framework for simulating and interpreting groundwater 14C age and δ13C

A reactive transport modeling framework is presented that allows simultaneous assessment of groundwater flow, water quality evolution including δ13C, and 14C activity or “age”. Through application of this framework, simulated 14C activities can be directly compared with measured 14C activities. This bypasses the need for interpretation of a 14C age prior to flow simulation through factoring out processes other than radioactive decay, which typically involves simplifying assumptions regarding spatial and temporal variability in reactions, flow, and mixing. The utility of the approach is demonstrated for an aquifer system with spatially variable carbonate mineral distribution, multiple organic carbon sources, and transient boundary conditions for 14C activity in the recharge water. In this case, the simulated 14C age was shown to be relatively insensitive to isotopic fractionation during DOC oxidation and variations in assumed DOC degradation behavior. We demonstrate that the model allows quantitative testing of hypotheses regarding controls on groundwater age and water quality evolution for all three carbon isotopes. The approach also facilitates incorporation of multiple environmental tracers and combination with parameter optimization techniques.

Impact of Data Density and Geostatistical Simulation Technique on the Estimation of Residence Times in a Synthetic Two-dimensional Aquifer

Connectivity patterns of heterogeneous porous media are important in the estimation of groundwater residence time distributions (RTDs). Understanding the connectivity patterns of a hydraulic conductivity (