Subsurface Stream Research Articles

Assessing karst landscape degradation based on the void development of karst aquifers in Gunungsewu, Indonesia

Compared to other landforms, karst areas are among those emerging from the dissolution process that have a higher risk of land degradation. The likelihood of karst landforms being harmed is increased by urbanization and other human activities like extensive agriculture. Subsurface streams' water quality gets worse when surface pollutants infiltrate through developed karst features like sinkholes and karst ponors. There is a greater risk of land degradation as more karst features, in this case void size, develop. The purpose of this research is to assess how void development, or the degree of karstification, relates to the potential for karst spring pollution in the event that land degradation occurs on the surface of the Karst Drainage System (KDS). This research was conducted at the KDS of Beton and Gremeng Spring in the Gunungsewu karst area, Indonesia. In addition, this study also provides recommendations related to environmental management on the basis of the level of development of voids at both sites. The degree of karstification represents the phase at which a hydrogeological system has been developing, and this information was later considered in formulating strategies for protecting karst groundwater from contamination. The results show that Beton and Gremeng had a complex discharge regime with degrees of karstification at 8 and 5.5, respectively. Based on flood hydrograph components, it was further confirmed that both areas were in the mature phase. The higher the degree of karstification, the higher the vulnerability to pollution.

A FCM Runoff Model for Small Rivers with Rainfall Recharge. 1. The Concept and Algorithms

The flood cycle model FCM is a lumped conceptual water-balance model designed to simulate rain runoff at a scale of a small river basin. The development of the FCM model includes the description of the dynamics of the major components of the total moisture reserves of a basin and reproduces the effect of spatial expansion of the drainage network through temporary surface and subsurface streams during extraordinary floods. The accepted conceptual assumptions of the model, which are in agreement with the rational hydrological considerations, lead to three runoff-formation regimes, referred to as intravolume, surface and breakthrough. The concept and algorithms of FCM are given in detail.

A reactivated cave system induces rapidly developing cover-collapse sinkholes in Tasmania, Australia

The reasons of rapid development of cover-collapse sinkholes near Railton, Tasmania were investigated after sinkholes first appeared in a mature pine plantation in 2011. Sinkhole monitoring, dye tracing, chemical analyses of stream water and field observations indicated that large sinkholes in Quaternary alluvium overlying Ordovician limestone resulted from deepening of a limestone quarry south of the plantation. Intersection of a cave system during quarrying lowered the local water table, initiated streamsinks, and caused streams that previously flowed north towards the Mersey River to flow south to the quarry, via streamsinks and reactivated subsurface channels. Concurrently, the thickness of the vadose zone in thick Quaternary sediments overlying the limestone increased, hydraulic support for the sediments decreased, and numerous cover-collapse sinkholes formed. Chemical data including isotope analyses confirmed that the local aquifer is mainly recharged after high winter rainfall during high stream flow, when a proportion of the water in surface streams also flows directly northwards to the Mersey River. During low (summer) flow most water in surface streams flows into streamsinks and south into the quarry via the cave system. It is then pumped to a surface stream running north through the area in question, returns into the cave system via streamsinks and sinkholes, and finally returns to the quarry. The study demonstrates that water-table lowering by quarrying in limestone is not confined to cones of depression but may extend for kilometres if relict subsurface stream channels are intersected. In this area, the rapid development of cover-collapse sinkholes presents risks to infrastructure (a railway line, an electricity transmission line, and roads); to stream biodiversity because of altered water flow duration and water quality; to the safety of forest contractors, particularly those using heavy machines; and to the future use of presently productive land for economic return.

The Forms of Linear Structure of Overland Flow in Medium-Height Mountain Regions: Case Study of the Sikhote Alin

Large subsurface streams on steep slopes have stable positions with distinct catchment areas (drainage basins), linear shapes, and lengths of tens and hundreds meters. They form a closed drainage network with a role and scale similar to the primary hydrographic network and serve as specific hydrological objects—drains. Drains are widespread in the medium-height humid mountain zone, where they account for the formation of a considerable portion of slope runoff. The hydrographic and hydrological characteristics of drains have been examined in small river basins of the Sikhote Alin with the use of field studies, hydrometric measurements, and large-scale topographic surveys.

Locating suitable sites for the construction of underground dams using the subsurface flow simulation (SWAT model) and analytical network process (ANP) (case study: Daroongar watershed, Iran)

Underground dams are constructions that prevent the natural flow of ground water and create underground water resources. Although lateral-flow volume is one of the main criteria to locate an underground dam, this factor is ignored in the most studies. In this research, the water balance equation was simulated using the SWAT model in the Daroongar watershed (northeastern Iran), and the paths of subsurface water flows were identified. Continuing by giving the weight to an 11-layer analytical network process (ANP), potential suitable locations for the construction of underground dams were determined. With the implementation of the SWAT model in the Daroongar watershed, R2, bR2, and NS (Nash Satklyf) coefficients were 0.77, 0.75, and 0.68, respectively, in the calibration phase; in the validation phase, they were 0.71, 0.67, and 0.61 in the same order. All these coefficients show that the efficiency of the SWAT model in simulating water balance is acceptable. Results of the sensitivity analysis of the SWAT model show 27 effective parameters, where the runoff curve number (CN2) was identified as the most important parameter. The results of the ANP model prove that subsurface flow, weighing 0.131, includes the most weight allocated. Results from the ANP model indicated that this is necessary for determining the extent of subsurface flow using the SWAT model. In addition, the reservoir volume and runoff volume, respectively, with a weight of 0.109 and 0.101 are more important than other factors. As a result, the greater the current subsurface stream, the more important it is in relation with other streams. The results showed that streams with ranks 3 and 4, which are located on slopes less than 15%, are suitable for the construction of underground dams.

Toward measuring biogeochemistry within the stream‐groundwater interface at the network scale: An initial assessment of two spatial sampling strategies

AbstractIt is important to understand how point measurements across spatially heterogeneous ecosystems are scaled to represent these systems. Stream biogeochemistry presents an illustrative example because water quality concerns within stream networks and recipient water bodies motivate heterogeneous watershed studies. Measurements of the stream water‐groundwater (SW‐GW) interface (i.e., the shallow stream subsurface) are well‐documented for point‐scale sampling density measurements (i.e., cm2–m2 features), but poorly characterized for network‐scale sampling density measurements (i.e., km2; stream reaches and networks). Sampling the SW‐GW interface is more time and labor intensive than surface water sampling, meaning sample point selection must be made with care for network‐scale analyses. In this study, we endeavor to determine which of two common spatial sampling schemes is appropriate for characterizing SW‐GW interface biogeochemistry across a third‐order stream network, focusing on dissolved organic carbon. The first scheme, called Local Sampling, focuses on characterizing small‐scale (< 10 m2) variability produced by the local physical and biogeochemical heterogeneity, with fewer points across the stream network. The second scheme, called Longitudinal Sampling, has approximately the same number of measurements distributed over many more points across the stream network with less local variability characterization. This comparison reveals that selection of a Local Sampling versus a Longitudinal Sampling scheme influences the biogeochemical pattern interpretation at the stream network scale. Additionally, this study found that increasing observation efforts at the local scale added limited information for reach‐ to network‐scale biogeochemical patterns, suggesting that emphasis should be placed on characterizing variability across broader spatial scales with the Longitudinal Sampling approach.

Vertical movements through subsurface stream sediments by benthic macroinvertebrates during experimental drying are influenced by sediment characteristics and species traits

Abstract Streambed drying is becoming more common due to climate change and increasing anthropogenic water resource pressures. Subsurface sediments are a potential refuge for benthic macroinvertebrates during drying events in temporary streams. Sediment characteristics are important controls on the distribution of macroinvertebrates in subsurface habitats, but difficulties making observations impedes quantification of vertical movements. Species traits (e.g. subsurface habitat affinity) also influence vertical movements of macroinvertebrates into the subsurface sediments, but most species‐specific responses remain uncharacterised. Transparent artificial mesocosms were used to directly observe the vertical movements of individuals of three aquatic insect nymphs and two crustaceans. Mixtures of three types of transparent sediment of varying particle size were combined to produce six treatments with differing interstitial pore volumes and, hence, differing subsurface porosity. Macroinvertebrate vertical movements were measured during incremental reductions in water level from 5 cm above to 20 cm below the sediment surface. These species comprised a variety of trait categories including feeding group, species affinity to temporary streams and subsurface habitats. Active and passive vertical movements were determined by conducting experiments with both live individuals and their cadavers. Sediment treatment influenced the vertical movements of individuals as reducing subsurface porosity decreased vertical movements for most species. Vertical movement into subsurface sediments in response to water level reduction was the result of active, not passive, movements for all species. Species identity influenced the vertical movements made by individuals. Nemoura cambrica had the highest affinity for temporary streams and subsurface habitats and its vertical movements were unaffected by sediment treatment, generally reaching depths between 20 and 25 cm. Most individuals of species with a weaker subsurface affinity (i.e. the benthic grazer Heptagenia sulphurea and the filter‐feeder Hydropsyche siltalai) became stranded as water levels were reduced in all sediment treatments. Vertical movements of Gammarus pulex and Asellus aquaticus were restricted primarily by pore volume, these taxa becoming stranded most commonly in sediments with smaller interstitial volumes. Our results highlight the need for the development and implementation of river management strategies that increase streambed porosity, allowing macroinvertebrates to access to the saturated subsurface habitat during stream drying.

Assessment of hydropower potential in small karst catchments: the case of the Rocche Plateau, Central Italy

Estimation of flow duration characteristics is key in assessing hydropower potential in natural catchments. However, such analysis is not usually straightforward, especially in ungauged sites and/or in complex catchment areas. In this study we evaluate the feasibility of revamping of a small hydroelectric power plant, located in a karst plateau in central Italy, by assessing the hydropower potential of its feeding surface and subsurface stream network. A thorough analysis of runoff processes occurring in the examined area is carried out in order to corroborate regionalization studies based on measured specific flows in neighboring homogeneous basins. The results show an appreciable availability of water resources to be exploited for hydropower purposes.

Modelling and experimental study of coupled exchange ofp,p′-DDE and colloids in a flume simulated stream-streambed system

Stream–subsurface exchange plays a significant role in the fate and transport of contaminants in streams. It has been modelled explicitly by considering fundamental processes such as hydraulic exchange, colloid filtration, and contaminant interactions with streambed sediments and colloids. The models have been successfully applied to simulate the transport of inorganic metals and nutrients. In this study, laboratory experiments were conducted in a recirculating flume to investigate the exchange of a hydrophobic organic contaminant, p,p′-dichloro-diphenyl-dichloroethane (DDE), between a stream and a quartz sand bed. A previously developed process-based multiphase exchange model was modified by accounting for the p,p′-DDE kinetic adsorption to and desorption from the bed sediments/colloids and was applied to interpret the experimental results. Model input parameters were obtained by conducting independent small-scale batch experiments. Results indicate that the immobilization of p,p′-DDE in the quartz sand bed can occur under representative natural stream conditions. The observed p,p′-DDE exchange was successfully simulated by the process-based model. The model sensitivity analysis results show that the exchange of p,p′-DDE can be sensitive to either the sediment sorption/desorption parameters or colloidal parameters depending on the experimental conditions tested. For the experimental conditions employed here, the effect of colloids on contaminant transport is expected to be minimal, and the stream–subsurface exchange of p,p′-DDE is dominated by the interaction of p,p′-DDE with bed sediment. The work presented here contributes to a better mechanistic understanding of the complex transport process that hydrophobic organic contaminants undergo in natural streams and to the development of reliable, predictive models for the assessment of impacted streams. Copyright © 2015 John Wiley & Sons, Ltd.

Near-surface internal melting: a substantial mass loss on Antarctic Dry Valley glaciers

AbstractThe McMurdo Dry Valleys, southern Victoria Land, East Antarctica, are a polar desert, and melt from glacial ice is the primary source of water to streams, lakes and associated ecosystems. Previous work found that to adequately model glacier ablation and subsurface ice temperatures with a surface energy-balance model required including the transmission of solar radiation into the ice. Here we investigate the contribution of subsurface melt to the mass balance of (and runoff from) Dry Valley glaciers by including a drainage process in the model and applying the model to three glacier sites using 13 years of hourly meteorological data. Model results for the smooth glacier surfaces common to many glaciers in the Dry Valleys showed that sublimation was typically the largest component of surface lowering, with rare episodes of surface melting, consistent with anecdotal field observations. Results also showed extensive internal melting 5–15 cm below the ice surface, the drainage of which accounted for ~50% of summer ablation. This is consistent with field observations of subsurface streams and formation of a weathering crust. We identify an annual cycle of weathering crust formation in summer and its removal during the 10 months of winter sublimation.

The Early Hercynian paleo-karstification in the Block 12 of Tahe oilfield, northern Tarim Basin, China

It is indicated in this paper that the karstification occurred in Block 12 of Tahe oilfield during Early Hercynian stage on the basis of identification features obtained through the latest research on seismology, petrology, and geochemistry. Distinguishable markers of the paleo-karstification mainly include: the lost strata and unconformable contact occurring between Lower–Middle Ordovician carbonates and Lower Carboniferous Bachu Formation, the occurrence of collapse breccias and fillings in subsurface stream, the evident difference between cave-fill calcite and matrix carbonate in the Sr/Ba ratio, and the log response for cavernous formations. The paleo-geomorphology of Early Hercynian karstification can be divided into four units: karst highland, I- and II-level karst slope, and karst basin in Tahe area, and the Block 12 was basically located in II-level karst slope. Three karst cycles developed in the Block 12 according to Early Hercynian tectonism characterized by multiphase pulse-type uplift movement and three sets of cavernous formation formed. II- and III-level cavernous formations mainly developed in Block 12. The Early Hercynian karstification is distributed on the north area of the pinch-out line of Upper Ordovician Sangtamu Formation in the plane, and is vertically developed in the Lower–Middle Ordovician carbonates. The karstification is essentially controlled by the paleo-geomorphology and fault. Fissures and caves formed by the karstification are the keys to affecting reservoir quality. The evolution model of Early Hercynian karstification has been established on the basis of the synthesis of the above-mentioned research results. It can be divided into three stages: rapidly uplifting and denudation stage, three episodes of uplifting and karstification, and subsiding and reworking stage.

Simulating the formation and fate of dense water in a midlatitude marginal sea during normal and warm winter conditions

AbstractDense shelf water production and the deep convection process in the Adriatic Sea are investigated, considering two case studies: the first is representative of the present climatic situation, whereas the second may be expected in a scenario characterized by mild winter conditions over the basin. Dense water production and spreading are studied using a high‐resolution implementation of the Massachusetts Institute of Technology general circulation model that is initialized and forced with realistic conditions. This paper provides qualitative and quantitative information on mass transport, dense water pathways, thermohaline structures, and the mixing properties of the basin. In the northern Adriatic shelf, seawater temperature is the key element for winter dense water production because it contributes more relevantly than salinity in determining density. In the southern Adriatic Sea, a small amount of dense water that cascades directly into the pit can be formed on the narrow western shelf only during cold winter conditions. Moreover, open ocean deepwater formation occurs in the middle of the southern basin. In late winter and spring, although only when winter conditions have been sufficiently cold, northern Adriatic dense shelf water forms a subsurface stream of which the densest part rapidly sinks in the southern pit along the shelf break, whereas its lighter part flows southward and reaches the Otranto Strait. The frequent occurrence of mild winter conditions could lead to lower dense water production, with a reduced dense water flow from the Adriatic Sea to the Ionian Sea and a potential great impact on the eastern Mediterranean thermohaline circulation.

Nitrogen Changes between Rural and Peri-Urban Stream Subsurface Waters (Yzeron Stream, France)

Urbanization subjects stream and groundwater to increased loads of organic nitrogen, nitrate, and ammonium. Therefore, studying nitrogen species at the urban stream-aquifer interface is important for water resource management. We report here results on water 18O/16O ratios and on nitrogen species in stream subsurface waters upstream and downstream of several combined sewer overflows (CSOs) in a rural area and peri-urban area, respectively. Water 18O/16O ratios were measured to trace the mixing of subsurface waters with stream channel water. Organic nitrogen (ON) and carbon (OC) slightly increased between rural and peri-urban environments in the cold season, but not in the warm season. The highest nitrate levels were observed in rural subsurface waters in the cold season. The lowest nitrate levels were found in peri-urban subsurface waters in the warm season; they corresponded to slow exchange of subsurface waters with channel water.

Mapping depth-to-clay using fitted multiple depth response curves of a proximal EMI sensor

As an alternative for the depth response approximations based on the theoretical Maxwell's equations, a procedure was proposed to fit depth response curves for different coil configurations. A 39 ha study area was selected in the Belgian loess belt, where loess material was situated on a Tertiary substrate. A survey with the DUALEM-21S electromagnetic induction instrument was carried out to map the depth-to-clay ( z clay). The depth response curves were fitted both for the vertical and perpendicular coil configurations using 85 depth observations of z clay. The resulting depth response curves R( z clay) were: R p, s ( z clay ) = 0 .8135 ⋅ e -1 .4131 ⋅ z clay s for the perpendicular coil configuration (with s as the intercoil spacing) and R v, s ( z clay ) = 0 .9802 ⋅ e -0 .8102 ⋅ z clay s for the vertical coil configuration. A set of 4 equations based on the developed depth response functions was used to model z clay at each of the 209 400 measurement points. These z clay predictions were validated using geo-electrical imaging. With two multi-electrode resistivity arrays, z clay was 1D-inverted at 95 locations along two transects, assuming a two-layered soil system. A coefficient of determination of 0.95, with a root mean-squared estimation error of 0.22 m, was found between the predicted and 1D-inverted depths. This procedure allowed the accurate 3D-reconstruction of the paleolandscape before the deposition of the loess. Flow lines were modelled on this paleosurface, revealing past or subsurface stream patterns not visible on the present relief.

Effects of overlying velocity, particle size, and biofilm growth on stream–subsurface exchange of particles

AbstractStream–subsurface exchange strongly influences the transport of contaminants, fine particles, and other ecologically relevant substances in streams. We used a recirculating laboratory flume (220 cm long and 20 cm wide) to study the effects of particle size, overlying velocity, and biofilm formation on stream–subsurface exchange of particles. Sodium chloride was used as a non‐reactive dissolved tracer and 1‐ and 5‐µm fluorescent microspheres were used as particulate tracers. Surface–subsurface exchange was observed with a clean sand bed and a bed colonized by an autotrophic–heterotrophic biofilm under two different overlying velocities, 0·9 and 5 cm s−1. Hydrodynamic interactions between the overlying flow and sand bed resulted in a reduction of solute and particle concentrations in the water column, and a corresponding accumulation of particles in both the sediments and in the biofilm. Increasing overlying velocity and particle size resulted in faster removal from the overlying water due to enhanced mass transfer to the bed. The presence of the biofilm did not affect solute exchange under any flow condition tested. The presence of the biofilm significantly increased the deposition of particles under an overlying velocity of 5 cm s−1, and produced a small but statistically insignificant increase at 0·9 cm−1. The particles preferentially deposited within the biofilm matrix relative to the underlying sand. These results demonstrate that hydrodynamic transport conditions, particle size, and biofilm formation play a key role in the transport of suspended particles, such as inorganic sediments, particulate organic matter, and pathogenic microorganisms in freshwater ecosystems, and should be taken into consideration when predicting the fate and transport of particles and contaminants in the environment. Copyright © 2009 John Wiley & Sons, Ltd.

Instream restoration: its effects on lateral stream–subsurface water exchange in urban and agricultural streams in Southern Ontario

AbstractInstream restoration strategies do not generally consider the subsurface environment. The study of recently restored stream reaches can provide an opportunity to assess the impacts of restoration on surface–subsurface exchange. In this study, lateral hyporheic zones occurring in a constructed gravel bar and re‐meandered stream reach were examined, using hydrometric data in combination with differences in background conservative ion and tracer injection experiments. Both the constructed gravel bar and the meander bends induced lateral hyporheic exchange flow. In the gravel bar, lateral hyporheic exchange increased after a riffle‐pool sequence was constructed in the channel adjacent to the bar. The substrate in the meander bends had low saturated hydraulic conductivity, and the stream–subsurface exchange was limited despite the large change in channel configuration. These results suggest that to enhance and maintain stream–subsurface water exchanges, restoration projects that modify horizontal geometry should involve construction of vertical morphologic features, and where floodplain sediments are fine‐grained, the addition of coarse sediments should also be incorporated in the design. Copyright © 2007 John Wiley & Sons, Ltd.

Subsurface flowpaths in a forested headwater stream harbor a diverse macroinvertebrate community

Headwater streams and wetlands with a combination of surface and subsurface flows are common features of many upland-forested watersheds. Unlike headwater stream reaches with continuous surface flow, the hydrology and ecology of subsurface stream reaches are poorly studied and not factored into existing wetland legislation. We assessed subsurface habitats and associated biota in a 435-m reach of a first-order, intermittent stream draining a riparian zone dominated by eastern hemlock (Tsuga canadensis) in north central Massachusetts. Stream flow was found only in subsurface flowpaths beneath large boulders and surface root mats over approximately 70% of the total stream length at summer base flow. Temperature, specific conductivity, dissolved oxygen, and dissolved organic carbon concentrations of subsurface water were similar to surface water. Macroinvertebrates were found in subsurface habitats but at a lower abundance and richness per unit area compared to surface habitats. Collectors such as Chironomidae, Polycentropodidae, and Ephemerellidae were generally the most abundant families in both surface and subsurface habitats. Our findings indicate that in some glaciated watersheds, intermittent streams with no visual evidence of surface flow may contain subsurface flowpaths with water chemistry and biota comparable to coupled perennial surface flow reaches. The prevalence and importance of subsurface habitats in some headwater streams may warrant review or revision of existing state and local regulatory definitions of intermittent and headwater streams.

Large-Scale Variation in Subsurface Stream Biofilms: A Cross-Regional Comparison of Metabolic Function and Community Similarity

We examined bacterial metabolic activity and community similarity in shallow subsurface stream sediments distributed across three regions of the eastern United States to assess whether there were parallel changes in functional and structural attributes at this large scale. Bacterial growth, oxygen consumption, and a suite of extracellular enzyme activities were assayed to describe functional variability. Community similarity was assessed using randomly amplified polymorphic DNA (RAPD) patterns. There were significant differences in streamwater chemistry, metabolic activity, and bacterial growth among regions with, for instance, twofold higher bacterial production in streams near Baltimore, MD, compared to Hubbard Brook, NH. Five of eight extracellular enzymes showed significant differences among regions. Cluster analyses of individual streams by metabolic variables showed clear groups with significant differences in representation of sites from different regions among groups. Clustering of sites based on randomly amplified polymorphic DNA banding resulted in groups with generally less internal similarity although there were still differences in distribution of regional sites. There was a marginally significant (p = 0.09) association between patterns based on functional and structural variables. There were statistically significant but weak (r2 approximately 30%) associations between landcover and measures of both structure and function. These patterns imply a large-scale organization of biofilm communities and this structure may be imposed by factor(s) such as landcover and covariates such as nutrient concentrations, which are known to also cause differences in macrobiota of stream ecosystems.

Applicability of the Transient Storage Model to the hyporheic exchange of metals

Stream–subsurface exchange results from a complex ensemble of transport mechanisms that require different modeling approaches. Field and laboratory experiments show that advective exchange through the underlying sediments is an important mechanism of solutes transport and storage in riverine systems. Here, Transient Storage Model parameters are obtained for reactive solute exchange driven by bedform-induced advection. Consideration of exchange induced by this single mechanism allows specific relationships between model parameters and system properties like solute reactivity to be identified. This work shows that when a simplified model like the Transient Storage Model is applied to analyze metal storage in river sediments, particular attention must be devoted to the choice of modeling parameters.

Flowpath Delineation and Ground Water Age, Allequash Basin, Wisconsin

AbstractAn analysis of ground water flowpaths to a lake and creek in northern Wisconsin shows the flow system in a geologically simple basin dominated by lakes can be surprisingly complex. Differences in source area, i.e., lakes or terrestrial, combined with the presence of intervening lakes, which may or may not capture underflowing ground water as water moves downgradient from recharge areas, contribute to a complex mix of flowpaths. The result is water of different chemistry and vastly different ages may discharge in close proximity. Flowpaths, travel times, and capture zones in the Allequash Basin in northern Wisconsin were delineated using particle tracking based on a calibrated steady‐state ground water flow model. The flowpath analysis supports the conclusions of Walker et al. (2003) who made inferences about flowpath characteristics from isotope and major ion chemistry. Simulated particle tracking agreed with Walker et al.'s measurements of water source (lake or terrestrial recharge) in the stream subsurface and also supported their assertion that ground water with a high calcium concentration in the lower basin of Allequash Lake is derived from long flowpaths. Numerical simulations show that ground water discharging in this area originates more than 5 km away in a source area located upgradient of Big Muskellunge Lake, which is upgradient of Allequash Lake. These results graphically illustrate that in settings with multiple sources of water with different age characteristics and converging flowlines (like the Allequash Basin) it may be difficult to obtain accurate estimates of ground water age by chemical analyses of ground water.