Assessing the importance of seepage and springs to nitrate flux in a stream network in the Wisconsin sand plains

Evaluating the flow paths that contribute to solute flux in stream networks can lead to greater understanding of the linkages between biogeochemistry and hydrology. We compared the contributions of groundwater in spring brooks and in seepage through the streambed to nitrate flux in the Emmons Creek network in the Wisconsin sand plains. We predicted that spring brooks would contribute disproportionately to nitrate flux due to the presumed higher advection rates in springs and less opportunity for nitrate removal relative to seeps. Nitrate flux was measured in 15 spring brooks that entered Emmons Creek. Nitrate flux from seepage was measured at the locations of 30 piezometers, based on Darcy's Law, and by a reach‐scale injection of Rhodamine water tracing (RWT). When seepage discharge was estimated from the RWT release, groundwater inputs from seepage and springs accounted for the discharge gain in the Emmons Creek channel. Springs brooks and seepage (based on the RWT release) contributed 37% and 63%, respectively, to nitrate flux inputs in the study reach. Contrary to our prediction, seeps contributed disproportionately to nitrate flux relative to their discharge. Relatively high rates of seepage discharge and higher than anticipated nitrate concentrations in the shallow pore water at seepage locations contributed to the unanticipated result.     

Intertidal spring dynamics and coastal nutrient loading revealed through geophysics,drone surveys,and in-situ monitoring

Coastal eutrophication poses an increasing risk to ecosystem health due to enhanced nutrient loading to the global coastline. Submarine groundwater discharge (SGD) represents a significant pathway for nitrate-nitrogen (NO₃-N) transport to the coast, but diffusive SGD transport is difficult to monitor directly, given the low flux rates and expansive discharge areas. In contrast, focused SGD from intertidal springs can potentially be sampled and directly gauged, providing unique insight into SGD and associated contaminant transport. Basin Head is a coastal lagoon in Prince Edward Island, Canada that is a federally protected ecosystem. Nitrate-nitrogen is conveyed from agricultural fields in the contributing watershed to the eutrophic lagoon via intertidal groundwater springs and groundwater-dominated tributaries. We used several field methods to characterize groundwater discharge, nutrient loading, and in-channel mixing associated with intertidal springs. The tributaries and intertidal springs were gauged and sampled to estimate a representative summer nitrate load to the lagoon. Our analysis revealed that NO₃-N export to the lagoon through tributaries and springs throughout summer 2023 was on average 401 kg N/month, with the combined spring loading comparable in magnitude to the combined tributary loading. We collected thermal infrared and visual imagery using drone surveys and found spatial overlap between cold-water plumes from the spring discharge and macroalgae blooms, indicating the local thermal and ecosystem impacts of the focused SGD. We also mapped the electrical resistivity (salinity) distribution in the water column around one large spring with electromagnetic geophysics at different tidal stages to reveal the three-dimensional spring plume dynamics. Results showed that the fresher spring water floated above the saline lagoon water with the brackish plume oriented in the direction of the tidal current. Collectively, our multi-pronged field investigations help elucidate the hydrologic, thermal, and nutrient dynamics of intertidal springs and the cascading ecosystem impacts.     

Analysis of karst aquifer spring flows with a gray system decomposition model

There are approximately 470,000 km(2) of karst aquifers that feed many large springs in North China. Turbulent flow often exists in these karst aquifers, which means that the classical ground water model based on Darcy's law cannot be applied here. Ground water data are rare for these aquifers. As a consequence, it is difficult to quantitatively investigate ground water flow in these karst systems. The purpose of this study is to develop a parsimonious model that predicts karst spring discharge using gray system theory. In this theory, a white color denotes a system that is completely characterized and a black color represents a system that is totally unknown. A gray system thus describes a complex system whose characteristics are only partially known or known with uncertainty. Using this theory, we investigated the karst spring discharge time series over different time scales. First, we identified three specific components of spring discharge: the long-term trend, periodic variation, and random fluctuation. We then used the gray system model to simulate the long-term trend and obtain periodic variation and random fluctuation components. Subsequently, we developed a predictive model for karst spring discharge. Application of the model to Liulin Springs, a representative example of karst springs in northern China, shows that the model performs well. The predicted results suggest that the Liulin Springs discharge will likely decrease over time, with small fluctuations.     

Etiology of Salinity and Water Origin,the Main Dilemma of Badab Sourt,a Unique Travertine Spring

Badab Sourt travertine‐depositing springs in the north of Iran, naturally create a unique surreal landscape containing a range of stepped travertine terraces, similarly found only in a few other places on earth. This site comprises of three travertine saline springs with different values of salinity and discharge (SP₁, SP₂, and SP₃) and one non‐travertine fresh karstic spring (SP₄) within a distance of about 300 m. The etiology behind this salinity and the water origin are the main research's dilemma that were investigated using geological, hydrochemical, and stable isotopic techniques. Based on the topography and isotopic results, the carbonate formations in northern (Khoshyeilagh and Mobarak) and southern (Cretaceous limestone) parts of the springs potentially provide the initial hydraulic gradient for deep circulation of the water and CO₂. However, geological studies indicate that the hydraulic connectivity of the Cretaceous formation to the travertine springs is interrupted by impermeable geological formations. Based on the proposed conceptual hydrogeological model and mass balance calculations, the SP₄ spring is locally recharged from the nearby karstic area of Khoshyeilagh formation through shallow, short and steep groundwater flow circulation that is completely different from the travertine springs. The travertine spring (SP₁) is recharged from more distant areas having higher altitudes on Mobarak and Khoshyeilagh limestone and circulate more deeply before emerging on the surface. The SP₂ and SP₃ springs can derive from the mixing of the saline water (SP₁) and fresh water (SP₄). The dissolution of interlayers of halite in Shemshak formation is concluded as the main source of salinity. This is the first research article in detail to survey hydrogeology of the travertine springs in Iran.     

Estimation of heat discharge rates using infrared measurements by a helicopter-borne thermocamera over the geothermal areas of Unzen volcano, Japan

Unzen volcano is situated on Shimabara Peninsula, western Kyushu, Japan. On the flank of the volcano, the Obama, Unzen and Shimabara hot springs are aligned in a direction from the southwest to the northeast across the peak. At Obama and Shimabara, heat is transferred mainly by water flow. But at Unzen heat is transferred by the discharge of natural steam and by conduction as well as water flow. In order to estimate the heat discharge by mechanisms other than water flow, infrared measurements by a helicopter-borne thermocamera were conducted over the Unzen hot spring area. The heat discharge was calculated from the thermal image by a method based on heat balance of the ground surface resulting in a value of 1.9 × 10⁶ cal/s (7.9 MW). The heat discharged by all mechanisms including that by water flow is estimated to be 5.0 × 10⁶ cal/s (21 MW). Similar preliminary estimates have been made for heat discharge at the Obama and Shimabara hot springs giving values of about 1.2 × 10⁷ cal/s (50 MW) and 1.0 × 10⁵ cal/s (0.4 MW), respectively. These values indicate that the heat discharge decreases with distance from the southwest to the northeast direction across the volcano. The total heat discharge from three hot spring areas on Unzen volcano is about 1.7 × 10⁷ cal/s (71 MW).The heat balance method appears useful for quantitative analysis of regional trends but its accuracy may not be always sufficient for detailed surveys. Several methods of determining heat flow, including the heat balance method, were compared at a test field in the Unzen hot spring area. The values obtained by the heat balance method coincide roughly with the other results but more detailed analysis is necessary to improve the accuracy of current methods of geothermal measurements.     

Variation of karst spring discharge in the recent five decades as an indicator of global climate change: A case study at Shanxi,northern China

As one of the largest international scientific pro- grams in geoscience and environmental science, global change studies were initiated in the early 1980s[1,2]. Noticeable achievements have been made in the stud- ies using indicators such as loess, marine sediment, permafrost, vermicular red earth, and even magmatic activity[2―6]. In recent years, the importance of ground- water as a new type of global change indicators has caused wide attention[7]. Stochastic, isotopic and hy- drochemical st…     

Modelling discharge through artesian springs based on a high‐resolution piezometric network

Artesian springs are localized aquifer outlets that originate when pressurized ground water is allowed to rise to the surface. Computing artesian discharge directly is often subject to practical difficulties such as restricted accessibility, abundant vegetation or slow flow rates. These circumstances call for indirect approaches to quantify flow. This paper presents a method to estimate ground water discharge through an upwelling spring by means of a three‐layer steady‐state groundwater flow model. Model inputs include on‐site measurements of vertical sediment permeability, sediment temperatures and hydraulic gradients. About 70 spring bed piezometers were used to carry out permeability tests within the spring sediments, as well as to quantify the hydraulic head at different depths below the discharge point. Sediment temperatures were measured at different depths and correlated to permeabilities in order to demonstrate the potential of temperature as a substitute for cumbersome slug tests. Results show that the spatial distribution of discharge through the spring bottom is highly heterogeneous, as sediment permeability varies by several orders of magnitude within centimetres. Sensitivity analyses imply that geostatistical interpolation is irrelevant to the results if field datasets come from a sufficiently high resolution of piezometric records. Copyright © 2013 John Wiley & Sons, Ltd.     

The integration of thermal infrared imaging,discharge measurements and numerical simulation to quantify the relative contributions of freshwater inflows to small estuaries in Atlantic Canada

Nutrient fluxes from developed catchments are often a significant factor in the declining water quality and ecological functioning in estuaries. Determining the relative contributions of surface water and groundwater discharge to nutrient‐sensitive estuaries is required because these two pathways may be characterized by different nutrient concentrations and temporal variability, and may thus require different remedial actions. Quantifying the volumetric discharge of groundwater, which may occur via diffuse seepage or springs, remains a significant challenge. In this contribution, the total discharge of freshwater, including groundwater, to two small nutrient‐sensitive estuaries in Prince Edward Island (Canada) is assessed using a unique combination of airborne thermal infrared imaging, direct discharge measurements in streams and shoreline springs, and numerical simulation of groundwater flow. The results of the thermal infrared surveys indicate that groundwater discharge occurs at discrete locations (springs) along the shoreline of both estuaries, which can be attributed to the fractured sandstone bedrock aquifer. The discharge measured at a sub‐set of the springs correlates well with the area of the thermal signal attributed to each discharge location and this information was used to determine the total spring discharge to each estuary. Stream discharge is shown to be the largest volumetric contribution of freshwater to both estuaries (83% for Trout River estuary and 78% for McIntyre Creek estuary); however, groundwater discharge is significant at between 13% and 18% of the total discharge. Comparison of the results from catchment‐scale groundwater flow models and the analysis of spring discharge suggest that diffuse seepage to both estuaries comprises only about 25% of the total groundwater discharge. The methods employed in this research provide a useful framework for determining the relative volumetric contributions of surface water and groundwater to small estuaries and the findings are expected to be relevant to other fractured sandstone coastal catchments in Atlantic Canada. Copyright © 2009 John Wiley & Sons, Ltd.     

Temporal cycles of karst denudation in northwest Georgia,U.S.A.

Time patterns of karst denudation in northwest Georgia (U.S.A.) were investigated at three spring sites for 12 months and at five stream sites for 10 years. Rainfall was evenly distributed and showed no significant seasonality. At the springs, as well as the streams, water hardness was largely controlled by discharge. At the springs, soil pCO₂ and water pH were strongly correlated (r + -0·69 to -0·83). Solute transport in spring waters was highly seasonal, with two conduit flow springs removing more limestone in the winter, and the diffuse flow spring removing more during the growing season. At the stream sites, most denudation occurred during the winter and spring seasons, and least during the summer. Fourier analysis showed that variations in denudation occur on deterministic (long-wave) as well as stochastic (shortwave) time scales. As contributing variables, discharge varied in short-wave and long-wave cycles, whereas soil pCO₂ showed only a long-wave cycle. The 12 month deterministic cycles were the most important, with changes in discharge taking precedence over soil pCO₂. Time series regression explains up to 69 per cent of changes in denudation through rain and soil pCO₂. Time cycles in available water are the key controlling factor of denudation, and amounts of available soil CO₂ may not be as important in the temporal patterns of karst downwearing as has been believed previously.     

Analytical and numerical models to explain steady rates of spring flow

Flow from some springs in former glacial lakebeds of the Upper Midwest is extremely steady throughout the year and does not increase significantly after precipitation events or seasonal recharge. Analytical and simplified numerical models of spring systems were used to determine whether preferential ground water flow through high-permeability features in shallow sandstone aquifers could produce typical values of spring discharge and the unusually steady rates of spring flow. The analytical model is based on a one-dimensional solution for periodic ground water flow. Solutions to this model suggest that it is unlikely that a periodic forcing due to seasonal variations in areal recharge would propagate to springs in a setting where high-permeability features exist. The analytical model shows that the effective length of the aquifer, or the length of flowpaths to a spring, and the total transmissivity of the aquifer have the greatest potential to impact the nature of spring flow in this setting. The numerical models show that high-permeability features can influence the magnitude of spring flow and the results demonstrate that the lengths of ground water flowpaths increase when high-permeability features are explicitly modeled, thus decreasing the likelihood for temporal variations in spring flow.     

Discharge estimation of submarine springs in the Dead Sea based on velocity or density measurements in proximity to the water surface

The Dead Sea is a closed lake, the water level of which is lowering at an alarming rate of about 1 m/year. Factors difficult to determine in its water balance are evaporation and groundwater inflow, some of which emanate as submarine groundwater discharge. A vertical buoyant jet generated by the difference in densities between the groundwater and the Dead Sea brine forms at submarine spring outlets. To characterize this flow field and to determine its volumetric discharge, a system was developed to measure the velocity and density of the ascending submarine groundwater across the center of the stream along several horizontal sections and equidistant depths while divers sampled the spring. This was also undertaken on an artificial submarine spring with a known discharge to determine the quality of the measurements and the accuracy of the method. The underwater widening of the flow is linear and independent of the volumetric spring discharge. The temperature of the Dead Sea brine at lower layers primarily determines the temperature of the surface of the upwelling, produced above the jet flow, as the origin of the main mass of water in the submarine jet flow is Dead Sea brine. Based on the measurements, a model is presented to evaluate the distribution of velocity and solute density in the flow field of an emanating buoyant jet. This model allows the calculation of the volumetric submarine discharge, merely requiring either the maximum flow velocity or the minimal density at a given depth.     

Quantifying the role of karstic groundwater in a snowmelt-dominated hydrologic system

River discharge in mountainous regions of the world is often dominated by snowmelt, but base flows are sustained primarily by groundwater storage and discharge. Although numerous recent studies have focused on base-flow discharge in mountain systems, almost no work has explicitly investigated the role of karst groundwater in these systems across a full range of flow conditions. We directly measured groundwater discharge from 48 karst springs in the Kaweah River and its five forks in the Sierra Nevada mountains, California, United States. Relationships between spring and river discharge showed that karst aquifers and springs provide significant storage and delayed discharge to the river. Regression models showed that, of all potential seasonal groundwater storage compartments in the river basin, the area of karst (0.1–4.4%) present provides the best explanation of base-flow recession in each fork of the Kaweah River (directly measured contributions from karst springs ranged from 3.5 to 16% during high-flow to 20 to 65% during base-flow conditions). These results show that, even in settings where karst represents a small portion of basin area, it may play an over-sized role in seasonal storage and water resources in mountain systems. Karst aquifers are the single most important non-snow storage component in the Kaweah River basin, and likely provide similar water storage capacities and higher base flows in other mountain river systems with karst when compared with systems without karst.     

Springflow hydrographs: eogenetic vs. telogenetic karst

Matrix permeability in the range of 10(-11) to 10(-14) m(2) characterizes eogenetic karst, where limestones have not been deeply buried. In contrast, limestones of postburial, telogenetic karst have matrix permeabilities on the order of 10(-15) to 10(-20) m(2). Is this difference in matrix permeability paralleled by a difference in the behavior of springs draining eogenetic and telogenetic karst? Log Q/Q(min) flow duration curves from 11 eogenetic-karst springs in Florida and 12 telogenetic-karst springs in Missouri, Kentucky, and Switzerland, plot in different fields because of the disparate slopes of the curves. The substantially lower flow variability in eogenetic-karst springs, which results in the steeper slopes of their flow duration curves, also makes for a strong contrast in patterns (e.g., "flashiness") between the eogenetic-karst and telogenetic-karst spring hydrographs. With respect to both spring hydrographs and the flow duration curves derived from them, the eogenetic-karst springs of Florida are more like basalt springs of Idaho than the telogenetic-karst springs of the study. From time-series analyses on discharge records for 31 springs and published time-series results for 28 additional sites spanning 11 countries, we conclude that (1) the ratio of maximum to mean (Q(max)/Q(mean)) discharge is less in springs of eogenetic karst than springs of telogenetic karst; (2) aquifer inertia (system memory) is larger in eogenetic karst; (3) eogenetic-karst aquifers take longer to respond to input signals; and (4) high-frequency events affect discharge less in eogenetic karst. All four of these results are consistent with the hypothesis that accessible storage is larger in eogenetic-karst aquifers than in telogenetic-karst aquifers.     

Using periodic hydrologic and geochemical sampling with limited continuous monitoring to characterize remote karst aquifers in the Kaweah River Basin,California, USA

Hydrogeologic field work in remote settings is often challenging: assessing spring behaviour and aquifer characteristics can be expensive in both time commitment and resources needed to assess these systems. In this study, we document the hydrology and geochemistry of 47 perennial karst springs in the Kaweah River, a mountain river basin in the Sierra Nevada, California. After preliminary hydrogeochemical characterization and grouping, selected springs were continuously monitored to further assess aquifer characteristics in each group. Later, in areas without previous dye‐tracing work, traces were conducted to establish connections between large sinking streams and springs. The springs have a wide range of inter‐spring and intra‐spring variability in discharge and geochemistry. We assessed this variability by performing statistical comparisons with spring chemistry and principal components analysis of all measured variables. Results show that springs can be divided into two distinct groups: high elevation springs of the Mineral King Valley and lower elevation springs throughout the rest of the basin. Continuous discharge, temperature and specific conductivity data from four springs (two from each group) were then used to characterize the hydrograph recession behaviour of springs in each group. Both groups showed statistically similar baseflow recession slopes, suggesting that both groups contain baseflow storage compartments with similar hydrogeologic properties. The biggest difference between each group is the variability in amount of water remaining in the aquifer during baseflow conditions. High elevation springs have lower baseflow discharges, relative to peak flow, than lower elevation springs, despite the fact that more precipitation falls at higher elevation. This is likely caused by differences in the amount of soil and epikarst storage, which are related to recent geomorphic events: high elevation aquifers were glaciated as recent as 41 thousand years ago (kya), while there is no evidence that low elevation aquifers were glaciated. As a result, lower elevations have developed thicker soils, weathered bedrock and epikarst. Copyright © 2016 John Wiley & Sons, Ltd.     

Delineating and quantifying ground water discharge zones using streambed temperatures

Streambed temperature mapping, hydraulic testing using minipiezometers, and geochemical analyses of interstitial water of the streambed were used to delineate the pattern of ground water discharge in a sandy streambed and to develop a flux-based conceptual model for ground water/surface water interactions. A new and simple empirical method was used to relate fluxes obtained from minipiezometer data to streambed temperatures. The relationship allowed flux to be calculated at locations where only streambed temperature measurements were made. Slug testing and potentiomanometer measurements at 34 piezometers indicated ground water discharge ranged from 0.03 to 446 L/m2/day (and possibly as high as 7060 L/m2/day) along a 60 m long by 11 to 14 m wide reach of river. Complex but similar plan-view patterns of flux were calculated for both summer and winter using hundreds of streambed temperatures measured on a 1 by 2 m grid. The reach was dominated by ground water discharge and 5% to 7% of the area accounted for approximately 20% to 24% of the total discharge. < 12% of the total area consisted of recharge zones or no-discharge zones. A conceptual model for ground water/surface water interactions consisting of five different behaviors was developed based on the magnitude and direction of flux across the surface of the streambed. The behaviors include short-circuit discharge (e.g., high-flow springs), high discharge (e.g., preferential flowpaths), low to moderate discharge, no discharge (e.g., horizontal hyporheic or ground water flow), and recharge. Geological variations at depth played a key role in determining which type of flow behavior occurred in the streambed.     

Nutrient flux, uptake, and transformation in a spring-fed stream in the Missouri Ozarks, USA

We examined nutrient flux, uptake, and transformation along a spring-fed stream in the Ozark region of Missouri, USA, over the year 2006. Water in Mill Creek originates from several springs, with a single spring contributing over 90% of the stream discharge during much of the year of study. Soluble reactive phosphate concentrations were usually low (<10 μg L⁻¹) along Mill Creek, but peaked during high discharge. Concentrations of dissolved inorganic nitrogen (DIN) were relatively high in the spring water, mainly as nitrate, but usually declined across a small pond and the 10-km length of Mill Creek. During low flows in summer and early autumn, the stream removed over 300 μg L⁻¹ of DIN over its 10-km length, or about 80% of the initial amount. DIN retention along the stream, as a percentage of the DIN upstream, was related mainly to discharge, with higher flows having much higher DIN concentrations. The net uptake rate of DIN uptake was 0.91 μg m⁻² s⁻¹ in the stream during summer baseflow. The uptake rate declined downstream for different reaches and was closely related to DIN concentration. In experimental channels, uptake by epilithic algae was one significant sink for nitrate-N in Mill Creek. In 2006, inorganic nutrient export during a single day after a spring storm was similar to export during 40–100 days of low flow conditions in summer and early autumn. Our results suggest that significant nutrient retention can occur during baseflow periods via biological uptake, whereas substantial export occurs during high flow conditions.     

Chemostatic behaviour of major ions and contaminants in a semiarid spring and stream system near Los Alamos,NM, USA

Recent studies have focused on the relationship between solute concentrations and discharge in streams, demonstrating that concentrations can vary little relative to changes in discharge (chemostatic behaviour). Chemostatic behaviour is dependent on catchment characteristics (e.g., lithology, geomorphology, and vegetation) and chemical characteristics of the solute (e.g., availability, reactivity, and mobility). An investigation of 3 springs and a stream near Los Alamos, NM, reveals that springs can behave in a chemostatic fashion as stream systems tend to do. Another unique finding of this study is that the anthropogenic contaminants barium and the high explosive RDX (hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine) can also behave chemostatically. The chemostatic behaviour of a contaminant has important implications for the residence time of contaminants in a system as well as having a major control on contaminant flux and mass transport. Redox (reduction–oxidation) and biogeochemically sensitive analytes (e.g., Fe, SO₄, and NO₃) display a combination of chemostatic and chemodynamic behaviour, showing the influence of temporally variable conditions on stream and spring chemistries.     

Interpretation of spring recession curves

Recession curves contain information on storage properties and different types of media such as porous, fractured, cracked lithologies and karst. Recession curve analysis provides a function that quantitatively describes the temporal discharge decay and expresses the drained volume between specific time limits (Hall 1968). This analysis also allows estimating the hydrological significance of the discharge function parameters and the hydrological properties of the aquifer. In this study, we analyze data from perennial springs in the Judean Mountains and from others in the Galilee Mountains, northern Israel. All the springs drain perched carbonate aquifers. Eight of the studied springs discharge from a karst dolomite sequence, whereas one flows out from a fractured, slumped block of chalk. We show that all the recession curves can be well fitted by a function that consists of two exponential terms with exponential coefficients alpha1 and alpha2. These coefficients are approximately constant for each spring, reflecting the hydraulic conductivity of different media through which the ground water flows to the spring. The highest coefficient represents the fast flow, probably through cracks, or quickflow, whereas the lower one reflects the slow flow through the porous medium, or baseflow. The comparison of recession curves from different springs and different years leads to the conclusion that the main factors that affect the recession curve exponential coefficients are the aquifer lithology and the geometry of the water conduits therein. In normal years of rainy winter and dry summer, alpha1 is constant in time. However, when the dry period is longer than usual because of a dry winter, alpha1 slightly decreases with time.     

Using Stormwater Hysteresis to Characterize Karst Spring Discharge

Discharge from karst springs contains a mixture of conduit and matrix water, but the variations in groundwater mixing are poorly known. Storm events present an opportunity to try to map flow components because water entering during storms is more dilute and provides a tracer as it mixes with pre‐event water along the flowpath from the recharge area to discharge at a spring. We used hysteresis plots of Mg/Ca ratios in a spring in the Cumberland Valley of Pennsylvania to map conduit (higher Ca) vs. diffuse (higher Mg) sources of recharge. We observed two types of temporal heterogeneity: within a storm event and from storm to storm. The timing of the variation in Mg/Ca suggested sources of mixing waters. An increase in the Mg/Ca ratio at the beginning of some storms while conductivity declined suggested diffuse recharge through the epikarst. The rapid changes in Mg/Ca ratios for low‐intensity events probably occurred as the rainfall waxed and waned and illustrate that a variety of flowpaths are available at this spring because additional flushing of Mg occurred. In contrast, the conductivity hysteresis began with dilute water initially and rotation was similar from storm to storm. Hysteresis plots of the Mg/Ca ratio have the potential of revealing more of the complexity in discharge than conductivity alone. A better understanding of flow components in karst is needed to protect these aquifers as a groundwater resource.     

Locating the zone of saline intrusion in a coastal karst aquifer using springflow data

Coastal fresh water aquifers are an increasingly desirable resource. In a karstic aquifer, sea water intrusion occurs as a salt water wedge, like in porous media. However, preferential flow conduits may alter the spatial and temporal distribution of the salt water. This is typically the case when the outlet of the aquifer is a brackish spring. This paper shows that salinity and flow rate variations at a spring, where salinity is inversely proportional to discharge, can help to understand the hydrodynamic functioning of the aquifer and to locate the fresh water-sea water mixing zone deep inside the aquifer. The volume of water-filled conduit between the sea water intrusion zone and the spring outlet is calculated by the integral over time of the flow rate during the time lag between the flow rate increase and the salinity decrease as measured at the spring. In the example of the spring at Almyros of Heraklio (Crete, Greece), this time lag is variable, depending on the discharge, but the volume of water-filled conduit appears to be constant, which shows that the processes of salt water intrusion and mixing in the conduit are constant throughout the year. The distance between the spring and the zone where sea water enters the conduit is estimated and provides an indication of the position where only fresh water is present in the conduit.