Specific Conductance Measurements Research Articles

Spatial Pattern Assessment and Prediction of Water and Sedimentary Mud Quality Changes in Lake Maurepas

Lake Maurepas, Louisiana, holds ecological, recreational, and economic significance, but recent concerns have arisen over its water quality due to industrial activities. From June to November 2023, we investigated water and sediment quality at nine sites and three depths. Results showed that NH3-N levels were within safety limits (0.11 ± 0.10 mg/L), while Total Nitrogen (TN, 0.83 ± 0.65 mg/L), Total Phosphorus (TP, 0.32 ± 0.13 mg/L), Chemical Oxygen Demand (COD, 25.94 ± 11.37 mg/L), Arsenic (As, 0.26 ± 0.17 mg/L), and Lead (Pb, 0.23 ± 0.002 mg/L) exceeded acceptable thresholds. Spatial-temporal analysis revealed significant variations across sites, depths, and sampling dates. Major contaminant sources included discharges from the Tickfaw, Amite, and Blind rivers, as well as a vehicle accident on Pass Manchac. Seismic and drilling activities by Air Products and Chemicals had little to no observed impact. Four AI algorithms were also evaluated using different physical parameter inputs to predict December’s chemical pollutant levels, which were missing due to adverse weather. The LSTM model outperformed the others, achieving R2 values of 0.852 for COD, 0.869 for TN, 0.842 for As, and 0.921 for TP and Pb. Predictions indicated decreasing pollutant levels in December, which matched salinity and specific conductance measurements, and reverted to those observed in September and October. This pattern is attributed to the settling of contaminants from the Pass Manchac accident and ongoing pollutant sources from September and October.

Dissolving the mystery of subsurface controls on snowmelt–discharge dynamics in karst mountain watersheds using hydrologic timeseries

AbstractStreamflow generation in mountain watersheds is strongly influenced by snow accumulation and melt as well as groundwater connectivity. In mountainous regions with limestone and dolomite geology, bedrock formations can host karst aquifers, which play a significant role in snowmelt–discharge dynamics. However, mapping complex karst features and the resulting surface‐groundwater exchanges at large scales remains infeasible. In this study, timeseries analysis of continuous discharge and specific conductance measurements were combined with gridded snowmelt predictions to characterize seasonal streamflow response and evaluate dominant watershed controls across 12 monitoring sites in a karstified 554 km2 watershed in northern Utah, USA. Immense surface water hydrologic variability across subcatchments, years and seasons was linked to geologic controls on groundwater dynamics. Unlike many mountain watersheds, the variability between subcatchments could not be well described by typical watershed properties, including elevation or surficial geology. To fill this gap, a conceptual framework was proposed to characterize subsurface controls on snowmelt–discharge dynamics in karst mountain watersheds in terms of conduit flow direction, aquifer storage capacity and connectivity. This framework requires only readily measured surface water and climatic data from nested monitoring sites and was applied to the study watershed to demonstrate its applicability for evaluating dominant controls and climate sensitivity.

Isotope Discrimination of Source Waters, Flowpaths, and Travel Times at an Acid-Generating, Lead–Zinc–Silver Mine, Silver Valley, Idaho, USA

Precipitation infiltrates into the lead–zinc–silver Bunker Hill Mine, oxidizes pyrite, and produces acidic waters that discharge from the mine portal. The metasedimentary geology and alteration from 100+ yr of mining provide a heterogeneous environment for source water infiltration and flow within the mine. A university–industry partnership was developed to trace the mine water sources, flowpaths, and travel times to identify potential areas for infiltration reduction. Snowpack, creek, and mine water samples were collected over a 1-year period for the analysis of δ2H, δ18O, and 3H, along with the in situ measurement of temperature, specific conductance, pH, dissolved oxygen, and flow. The isotope tracers were used to identify the source waters, unmix mine water as it moved deeper in the mine, and examine flowpaths in and near the acid-generating pyritic zone. The results indicate creek water infiltrating relatively quickly through the anthropogenically-modified pathways and causing the largest amount of acidic water in the upper levels of the mine. Slower, natural pathways associated with faults, fractures, and bedding planes produce mostly neutral waters with the source waters typically originating at higher elevations. Travel times ranged from <1 to 22 years with shorter pathways to the upper levels of the mine and increasing contributions deeper in the mine from pathways containing older, higher-elevation snowmelt. These slower and older inflows were identified by depleted δ18O values, smaller 3H concentrations, the dampening of the variability of the isotope signals, and pH increases. Reduction of infiltration zones near the upper workings of the mine likely will decrease the acidic waters in the upper levels of the mine, but the higher elevation infiltration zones will continue to contribute snowmelt-derived waters at all mine levels.

Application of the practical salinity scale to the waters of San Francisco estuary

The Practical Salinity Scale 1978 (PSS-78)—developed using samples of standard seawater—is widely used as a conductivity-based measure of salinity in oceans and estuaries. Here we evaluate the application of the scale across San Francisco Estuary where salinity reflects a mixture of seawater, riverine inflows, and agricultural return flows from islands within the estuary. We employ an extensive salinity data set that includes measurements of specific electrical conductance (EC) and major ion concentrations to demonstrate the scale's applicability in this estuary. We find the scale to be valid in waters dominated by seawater intrusion as well as in waters dominated by the largest riverine input to the estuary, i.e. the Sacramento River. In these waters, we further find that the scale to be valid well below its recommended lower-bound value of 2.0. However, the scale under-estimates salinity in waters dominated by the San Joaquin River, the second largest riverine input to the estuary. Similarly, the scale under-estimates salinity in agricultural return flows to the estuary. This work shows that a singular PSS-78 relationship cannot be used to accurately compute salinity across the entire estuary from EC measurements—a finding of considerable importance for salinity monitoring and modeling applications. Using data from the estuary, we propose appropriate corrections to the scale for these drainage-influenced waters. We recommend further research into how these modified PSS-78 formulations can be used to more accurately estimate salinity and ionic constituent concentrations in this and other estuaries.

Multi‐year high‐frequency sampling provides new runoff and biogeochemical insights in a discontinuous permafrost watershed

AbstractPermafrost‐underlain watersheds in the subarctic are sensitive to warming as small changes in ground thermal status will alter all components of the hydrological cycle. Globally, observed increases in winter flows and shifting water chemistry have most often been ascribed to permafrost thaw and deepening runoff pathways. However, there remain few studies in headwater catchments that examine coupled flow‐chemistry relations at high frequency over multiple years and seasons to evaluate the implications of environmental change. In this study, we use multi‐year high‐frequency measurement of discharge, specific conductance (SpC) and chromophoric dissolved organic matter (CDOM) along with traditional grab sampling of major ions to understand the sources and pathways of water and evaluate how distinct solutes are mobilized in a well‐studied subarctic basin in Yukon, Canada. Seasonally, the catchment exhibited considerable hysteresis in flow‐solute relations and had both chemostatic and dilution SpC–Q patterns with respect to major ions depending upon season and mobilization CDOM–Q signals. Storm events were extracted from high‐frequency data and normalized C–Q indices were determined and related to flow, catchment and meteorological variables. CDOM–Q events predominantly had an anti‐clockwise hysteresis and increases in DOC concentrations during storms, with some exception in the spring and fall. Conversely, SpC–Q events exhibited clockwise hysteresis and a dilution behaviour during events with less seasonal or inter‐annual variability. Information from this study supports previous conceptual models of thermally regulated runoff generation in a layered soil profile, yet also points to the importance of lateral connectivity and distal sources of solutes.

Application of anionic-nonionic mixed micellar system for solubilization of methylene blue dye

Present study highlights the interaction of methylene blue (MB), a cationic dye, with anionic surfactant, sodium dodecyl sulfate (SDS), and its mixed micelles with a nonionic surfactant Triton X-114 (TX-114). The results obtained from UV–visible spectrophotometry were used to obtain the degree of solubilization in terms of binding constant (Kb) and partition coefficient (Kx) of dye in single and mixed micellar systems (MMS). Gibb’s free energies of partitioning and binding were also calculated for both systems. Specific conductance was measured to determine the CMC of single and mixed micellar media at five different temperatures, T = (293.15 – 333.15) K. Temperature-dependent thermodynamic parameters like Gibb’s free energy of micellization (ΔGomic), enthalpy of micellization (ΔHomic), and entropy of micellization (ΔSomic) were calculated using data of specific conductance measurements. Effect of concentration of TX-114 on Kx and Kb values of the MB in MMS is also investigated. Spectroscopic and conductometric results revealed that the anionic-nonionic mixed micellar system was better and more effective for the removal and solubilization of MB from aqueous system as compared to anionic micelles only. However, it has been observed that after a critical concentration an increase of non-ionic surfactant concentration has a reverse effect on solubilizing power of ionic-nonionic mixed micellar system. This indicates that the dye-surfactant interaction became weaker due to possible decrease in ionic character of mixed micelles when the high fraction of TX-114 was used.

Technical note: Using long short-term memory models to fill data gaps in hydrological monitoring networks

Abstract. Quantifying the spatiotemporal dynamics in subsurface hydrological flows over a long time window usually employs a network of monitoring wells. However, such observations are often spatially sparse with potential temporal gaps due to poor quality or instrument failure. In this study, we explore the ability of recurrent neural networks to fill gaps in a spatially distributed time-series dataset. We use a well network that monitors the dynamic and heterogeneous hydrologic exchanges between the Columbia River and its adjacent groundwater aquifer at the U.S. Department of Energy's Hanford site. This 10-year-long dataset contains hourly temperature, specific conductance, and groundwater table elevation measurements from 42 wells with gaps of various lengths. We employ a long short-term memory (LSTM) model to capture the temporal variations in the observed system behaviors needed for gap filling. The performance of the LSTM-based gap-filling method was evaluated against a traditional autoregressive integrated moving average (ARIMA) method in terms of error statistics and accuracy in capturing the temporal patterns of river corridor wells with various dynamics signatures. Our study demonstrates that the ARIMA models yield better average error statistics, although they tend to have larger errors during time windows with abrupt changes or high-frequency (daily and subdaily) variations. The LSTM-based models excel in capturing both high-frequency and low-frequency (monthly and seasonal) dynamics. However, the inclusion of high-frequency fluctuations may also lead to overly dynamic predictions in time windows that lack such fluctuations. The LSTM can take advantage of the spatial information from neighboring wells to improve the gap-filling accuracy, especially for long gaps in system states that vary at subdaily scales. While LSTM models require substantial training data and have limited extrapolation power beyond the conditions represented in the training data, they afford great flexibility to account for the spatial correlations, temporal correlations, and nonlinearity in data without a priori assumptions. Thus, LSTMs provide effective alternatives to fill in data gaps in spatially distributed time-series observations characterized by multiple dominant frequencies of variability, which are essential for advancing our understanding of dynamic complex systems.

Dynamics of natural discharge of the hydrothermal system and geyser eruption regime in the Valley of Geysers, Kamchatka

The total deep component natural mass discharge Qd (defined in terms of chloride mass discharge Qcl) of the Valley of Geysers, on an average ranges from 280 kg/s from 1961 to 1984, to 230 kg/s after 2015. Post 2012, discharge measurements reveal a seasonal variation: the discharge increases (340–370 kg/s) during the winter-frozen period, and decreases during the summer flooding period (≈100 kg/s). Long term annually averaged Qd is 274 kg/s, Qcl is 0.247 kg/s and heat flow is 265 MW. In the course of using high-frequency (min−1) observations specific conductance measurements from 2017 to 2020, the total natural discharge was found to be cycling due to the internal cycling of geysers. It was mostly sensitive to the Bolshoy and Velikan geysers with averaged intervals between eruptions of 60 min and 70 min, respectively. The tracer chloride method estimates the volume of hot water erupted from geysers. This method yielded the following estimates: 5–34 m3 of hot water cyclically erupted from the Bolshoy geyser; 0.5–4.5 m3 erupted from the Velikan geyser between 2018 and 2020, and 24–144 m3 erupted before the 2014 mud-flow disaster; and 289–330 m3 erupted from the Grot geyser between May–June 2012. The seasonal features of natural discharge mentioned above may be explained in terms of cold-water infiltration into a two-phase geyser geothermal reservoir, especially if gas-phase condensation induces vacuum conditions, which may further reduce some thermal discharge features.

Spatial and Temporal Variability in Concentration‐Discharge Relationships at the Event Scale

AbstractThe analysis of concentration‐discharge (C‐Q) relationships from low‐frequency observations is commonly used to assess solute sources, mobilization, and reactive transport processes at the catchment scale. High‐frequency concentration measurements are increasingly available and offer additional insights into event‐scale export dynamics. However, only few studies have integrated inter‐annual and event‐scale C‐Q relationships. Here, we analyze high‐frequency measurements of specific conductance (EC), nitrate (NO3‐N) concentrations and spectral absorbance at 254 nm (SAC254, as a proxy for dissolved organic carbon) over a two year period for four neighboring catchments in Germany ranging from more pristine forested to agriculturally managed settings. We apply an integrated method that adds a hysteresis term to the established power law C‐Q model so that concentration intercept, C‐Q slope and hysteresis can be characterized simultaneously. We found that inter‐event variability in C‐Q hysteresis and slope were most pronounced for SAC254 in all catchments and for NO3‐N in forested catchments. SAC254 and NO3‐N event responses in the smallest forested catchment were closely coupled and explainable by antecedent conditions that hint to a common near‐stream source. In contrast, the event‐scale C‐Q patterns of EC in all catchments and of NO3‐N in the agricultural catchment without buffer zones around streams were less variable and similar to the inter‐annual C‐Q relationship indicating a homogeneity of mobilization processes over time. Event‐scale C‐Q analysis thus added key insights into catchment functioning whenever the inter‐annual C‐Q relationship contrasted with event‐scale responses. Analyzing long‐term and event‐scale behavior in one coherent framework helps to disentangle these scattered C‐Q patterns.

Evaluating Streamwater Dissolved Organic Carbon Dynamics in Context of Variable Flowpath Contributions With a Tracer‐Based Mixing Model

AbstractThis study focuses on characterizing the contributions of key terrestrial pathways that deliver dissolved organic carbon (DOC) to streams during hydrological events and on elucidating factors governing variation in water and DOC fluxes from these pathways. We made high‐frequency measurements of discharge, specific conductance (SC), and fluorescent dissolved organic matter (FDOM) during 221 events recorded over 2 years within four Vermont (USA) watersheds that range in area from 0.4 to 139 km2. Using the SC measurements, together with statistical information on discharge, we separated the event hydrographs into contributions from three terrestrial pathways, which we refer to as riparian quickflow, subsurface quickflow, and slow‐flow groundwater. The pathway discharges were used as input to a mixing model that closely approximated sub‐hourly streamwater DOC concentrations as measured with the FDOM sensors. Subsurface quickflow, comprised of pre‐event water, was the leading contributor to streamwater DOC fluxes, while riparian quickflow, comprised of event water, was the second‐leading contributor to streamwater DOC fluxes, despite comprising the smallest proportion of streamflow yield among the three end‐member pathways. Fixed‐effects regression analysis revealed that the relationship between DOC fluxes from the end‐member pathways and event magnitude was consistent across the four watersheds. This analysis also showed that DOC fluxes from the quickflow pathways increased significantly with temperature and varied inversely, but weakly, with catchment antecedent wetness. We believe that our approach, which leverages in‐stream sensors that enable high‐frequency measurements over extended periods, may be applicable for evaluating controls on DOC export from other watersheds within and beyond our study region.

Surface water‐groundwater exchange dynamics in buried‐valley aquifer systems

AbstractGroundwater is a primary source of drinking water worldwide, but excess nutrients and emerging contaminants could compromise groundwater quality and limit its usage as a drinking water source. As such contaminants become increasingly prevalent in the biosphere, a fundamental understanding of their fate and transport in groundwater systems is necessary to implement successful remediation strategies. The dynamics of surface water‐groundwater (hyporheic) exchange within a glacial, buried‐valley aquifer system are examined in the context of their implications for the transport of nutrients and contaminants in riparian sediments. High conductivity facies act as preferential flow pathways which enhance nutrient and contaminant delivery, especially during storm events, but transport throughout the aquifer also depends on subsurface sedimentary architecture (e.g. interbedded high and low conductivity facies). Temperature and specific conductance measurements indicate extensive hyporheic mixing close to the river channel, but surface water influence was also observed far from the stream‐aquifer interface. Measurements of river stage and hydraulic head indicate that significant flows during storms (i.e., hot moments) alter groundwater flow patterns, even between consecutive storm events, as riverbed conductivity and, more importantly, the hydraulic connectivity between the river and aquifer change. Given the similar mass transport characteristics among buried‐valley aquifers, these findings are likely representative of glacial aquifer systems worldwide. Our results suggest that water resources management decisions based on average (base) flow conditions may inaccurately represent the system being evaluated, and could reduce the effectiveness of remediation strategies for nutrients and emerging contaminants.

Drone-based imaging to assess the microbial water quality in an irrigation pond: A pilot study.

Microbial water quality datasets are essential in irrigated agricultural practices to detect and inform measures to prevent the contamination of produce. Escherichia coli (E. coli) concentrations are commonly used to evaluate microbial water quality. Remote sensing imagery has been successfully used to retrieve several water quality parameters that can be determinants of E. coli habitats in waterbodies. This pilot study was conducted to test the possibility of using imagery from a small unmanned aerial vehicle (sUAV or drone) to improve the estimation of microbial water quality in small irrigation ponds. In situ measurements of pH, turbidity, specific conductance, and concentrations of dissolved oxygen, chlorophyll-a, phycocyanin, and fluorescent dissolved organic matter were taken at depths of 0-15 cm in 23 locations across a pond in Central Maryland, USA. The pond surface was concurrently imaged using a drone with three modified GoPro cameras, and a multispectral MicaSense RedEdge camera with five spectral bands. The GoPro imagery was decomposed into red, blue, and green components. Mean digital numbers for 1-m radius areas in the images were combined with the water quality data to provide input for a regression tree-based analysis. The accuracy of the regression-tree data description with "only imagery" inputs was the same or better than that of trees constructed with "only water-quality parameters" as inputs. From multiple cross-validation runs with "only imagery" inputs for the regression trees, the average (±SD) determination coefficient and root-mean-squared error of the decimal logarithm of E. coli concentrations were 0.793 ± 0.035 and 0.131 ± 0.011, respectively. The results of this study demonstrate the opportunities for using sUAV imagery for obtaining a more accurate delineation of the spatial variation of E. coli concentrations in irrigation ponds.

Restriction of lung volumes but normal function of pulmonary tissue in mulibrey nanism

Mulibrey nanism (MUL) is a rare growth restriction disorder with multiple organ manifestations caused by genetic defects affecting the TRIM37 protein. A perimyocardial heart disease is the most serious manifestation. Many MUL children appear to suffer from airway obstruction related to infection or exercise, prompting use of inhaled therapies. Asthma medication is continued up to adolescence or even to adulthood due to persisting of symptoms. The pulmonary pathophysiology has previously not been evaluated in any MUL cohort. Thirty three finnish MUL patients (median age 20 years) were investigated with several lung function tests: spirometry with bronchodilatation test, single-breath diffusing capacity for carbon monoxide, single-breath lung volume measurements with helium dilution, and thoracic gas volume, airway resistance and specific conductance measurements with a body plethysmograph. As MUL typically affects body proportions, all variables were compared with reference values and with predicted values calculated from sitting height. Total lung capacity and forced vital capacity were markedly reduced (total lung capacity [TLC] and forced vital capacity [FVC], P < .001, 51%-63% of predicted) and also forced expiratory volume in the first second was reduced (FEV1; P < .001, 47%-57%). No signs of airway obstruction was seen (normal FEV1/FVC and specific airway conductance SGaw). Diffusing capacity (DLCO) was decreased (P < .001, 60%-67%) but when related to alveolar volume it was increased (DLCO/VA, P < .001, 130%-148%). Bronchodilatation suggesting active asthma (FEV1 change ≥12% and ≥​​200 mL) was found only in one patient. MUL patients typically have volume restriction of the lungs, but function of the pulmonary tissue remains intact. Evidence of asthma in lung function testing at adult age is rare.

Selenium dynamics in headwater streams of the central Appalachian coalfield.

Coal mining can cause selenium (Se) contamination in US Appalachian streams, but linkages between water-column Se concentrations and Se bioaccumulation within Appalachian headwater streams have rarely been quantified. Using elevated specific conductance (SC) in stream water as an indicator of mining influence, we evaluated relationships between SC and Se concentrations in macroinvertebrates and examined dynamics of Se bioaccumulation in headwater streams. Twenty-three Appalachian streams were categorized into 3 stream types based on SC measurements: 1) reference streams with no coal-mining history; 2) mining-influenced, high-SC streams; and 3) mining-influenced, low-SC streams. Selenium concentrations in macroinvertebrates exhibited strong positive associations with both SC and dissolved Se concentrations in stream water. At 3 streams of each type, we further collected water, particulate matter (sediment, biofilm, leaf detritus), and macroinvertebrates and analyzed them for Se during 2 seasons. Enrichment, trophic transfer, and bioaccumulation factors were calculated and compared among stream types. Particulate matter and macroinvertebrates in mining-influenced streams accumulated high Se concentrations relative to reference streams. Concentrations were found at levels indicating Se to be a potential environmental stressor to aquatic life. Most Se enrichment, trophic transfer, and bioaccumulation factors were independent of season. Enrichment factors for biofilm and sediments and bioaccumulation factors for macroinvertebrate predators varied negatively with water-column Se. Our results increase scientific understanding of Se bioaccumulation processes in Appalachian headwater streams. Environ Toxicol Chem 2018;37:2714-2726. © 2018 SETAC.

Evidence of Possible Recharge Zones for Lake Salda (Turkey)

This study explores the evidence of recharge locations using hydrogeochemical and physicochemical measurements in an alkaline lake, Lake Salda, in Burdur, Turkey. In-situ measurements have been performed using a conductivity–temperature–depth device to map the physicochemical dynamic of the lake. Water and sediment samples were collected on the surface and floor of the lake. A seismic study was also carried out in order to observe the geometry of the lake floor. In addition, thermal distribution was mapped using the thermal band of Landsat 7 ETM+ and Landsat 8 satellite images. Temperature and specific conductance measurements were mapped using a new technique, Empirical Bayesian Kriging (EBK), from the lake’s surface to the floor. According to interpolation maps obtained from the EBK, possible water inputs were observed close to a fault at the south-eastern part of the lake. The results of thermal band imaging also reveal the probability of a fault effecting the recharge on the surface. The results of water and sediment samples present a richness in Mg2+ and Fe2+ elements respectively on the floor of the lake. Finally, seismic results show some possible recharge zones on the floor of the lake, and sediment results indicate that there should be peridotite occurrence below the alluvium unit.

Direct Evidence of Meltwater Flow Within a Firn Aquifer in Southeast Greenland

AbstractWithin the lower percolation zone of the southeastern Greenland ice sheet, meltwater has accumulated within the firn pore space, forming extensive firn aquifers. Previously, it was unclear if these aquifers stored or facilitated meltwater runoff. Following mixing of a saline solution into boreholes within the aquifer, we observe that specific conductance measurements decreased over time as flowing freshwater diluted the saline mixture in the borehole. These tests indicate that water flows through the aquifer with an average specific discharge of 4.3 × 10−6 m/s (σ = 2.5 × 10−6 m/s). The specific discharge decreases dramatically to 0 m/s, defining the bottom of the aquifer between 30 to 50 m depth. The observed flow indicates that the firn pore space is a short‐term (&lt;30 years) storage mechanism in this region. Meltwater flows out of the aquifer, likely into nearby crevasses, and possibly down to the base of the ice sheet and into the ocean.

Seasonal pattern of anthropogenic salinization in temperate forested headwater streams

Salinization of freshwaters by human activities is of growing concern globally. Consequences of salt pollution include adverse effects to aquatic biodiversity, ecosystem function, human health, and ecosystem services. In headwater streams of the temperate forests of eastern USA, elevated specific conductance (SC), a surrogate measurement for the major dissolved ions composing salinity, has been linked to decreased diversity of aquatic insects. However, such linkages have typically been based on limited numbers of SC measurements that do not quantify intra-annual variation. Effective management of salinization requires tools to accurately monitor and predict salinity while accounting for temporal variability. Toward that end, high-frequency SC data were collected within the central Appalachian coalfield over 4 years at 25 forested headwater streams spanning a gradient of salinity. A sinusoidal periodic function was used to model the annual cycle of SC, averaged across years and streams. The resultant model revealed that, on average, salinity deviated approximately ±20% from annual mean levels across all years and streams, with minimum SC occurring in late winter and peak SC occurring in late summer. The pattern was evident in headwater streams influenced by surface coal mining, unmined headwater reference streams with low salinity, and larger-order salinized rivers draining the study area. The pattern was strongly responsive to varying seasonal dilution as driven by catchment evapotranspiration, an effect that was amplified slightly in unmined catchments with greater relative forest cover. Evaluation of alternative sampling intervals indicated that discrete sampling can approximate the model performance afforded by high-frequency data but model error increases rapidly as discrete sampling intervals exceed 30 days. This study demonstrates that intra-annual variation of salinity in temperate forested headwater streams of Appalachia USA follows a natural seasonal pattern, driven by interactive influences on water quantity and quality of climate, geology, and terrestrial vegetation. Because climatic and vegetation dynamics vary annually in a seasonal, cyclic manner, a periodic function can be used to fit a sinusoidal model to the salinity pattern. The model framework used here is broadly applicable in systems with streamflow-dependent chronic salinity stress.

Electrical resistivity dynamics beneath a fractured sedimentary bedrock riverbed in response to temperature and groundwater–surface water exchange

Abstract. Bedrock rivers occur where surface water flows along an exposed rock surface. Fractured sedimentary bedrock can exhibit variable groundwater residence times, anisotropic flow paths, and heterogeneity, along with diffusive exchange between fractures and rock matrix. These properties of the rock will affect thermal transients in the riverbed and groundwater–surface water exchange. In this study, surface electrical methods were used as a non-invasive technique to assess the scale and temporal variability of riverbed temperature and groundwater–surface water interaction beneath a sedimentary bedrock riverbed. Conditions were monitored at a semi-daily to semi-weekly interval over a full annual period that included a seasonal freeze–thaw cycle. Surface electromagnetic induction (EMI) and electrical resistivity tomography (ERT) methods captured conditions beneath the riverbed along a pool–riffle sequence of the Eramosa River in Canada. Geophysical datasets were accompanied by continuous measurements of aqueous specific conductance, temperature, and river stage. Time-lapse vertical temperature trolling within a lined borehole adjacent to the river revealed active groundwater flow zones along fracture networks within the upper 10 m of rock. EMI measurements collected during cooler high-flow and warmer low-flow periods identified a spatiotemporal riverbed response that was largely dependent upon riverbed morphology and seasonal groundwater temperature. Time-lapse ERT profiles across the pool and riffle sequence identified seasonal transients within the upper 2 and 3 m of rock, respectively, with spatial variations controlled by riverbed morphology (pool versus riffle) and dominant surficial rock properties (competent versus weathered rock rubble surface). While the pool and riffle both exhibited a dynamic resistivity through seasonal cooling and warming cycles, conditions beneath the pool were more variable, largely due to the formation of river ice during the winter season. We show that surface electrical resistivity methods have the capacity to detect and resolve electrical resistivity transience beneath a fractured bedrock riverbed in response to porewater temperature and specific conductance fluctuations over a complete annual cycle.

Flowpath and retention of snowmelt in an ice‐covered arctic lake

AbstractThe extent to which snowmelt flowing into ice‐covered lakes spreads horizontally and mixes vertically influences retention of solutes derived from the landscape. To quantify these transport processes and retention, we combine time series temperature and specific conductance measurements in Toolik Lake (Alaska) and its major inflow, with measurements of discharge and meteorology, and profiles of specific conductance, temperature, fluorescence, chlorophyll a, and dissolved organic carbon (DOC) in spring of 3 yr. During early snowmelt, the concentration of DOC in the stream was 750 μM, twice that in the lake. During slow melt (discharge (Q) &lt; 4 m3 s−1), the incoming solute‐rich intrusion spread lakewide below the ice. During melt with Q &gt; 6 m3 s−1, the incoming water partially flushed the inlet basin and the more dilute water flowed over the original intrusion with a preferential flowpath to the outlet. Penetrative convection was restricted by the increased density gradients from the incoming plume and initially constrained to shallow mixing zones associated with the step changes in density. As ice thickness decreased to less than 1 m, heating caused density instabilities at the base of the intrusions that mixed solutes ∼ 10 m vertically, contributing to retention. Near‐surface layers enriched with DOC persisted for ∼ 10 d during a rapid melt and for over 3 weeks when the melt was slow. Retention, of order 10–20%, also depended on the rapidity of melt and magnitude of discharge.

Thermal groundwater contributions of arsenic and other trace elements to the middle Provo River, Utah, USA

Groundwater inputs can impact river water quality but are difficult to disentangle from agricultural, urban, and storm runoff. To better understand the multiple processes affecting water quality, we used major solute and trace element concentrations with continuous measurements of flow rates and specific conductance to track temporal and spatial changes in surface water and groundwater solute inputs into the middle Provo River, located in northern Utah, USA. Thermal groundwater was the most important source of major solutes and trace elements to the middle Provo River, with concentrations of As, B, Cs, Li, Sr, and Rb increasing dramatically (twofold to tenfold) downstream of thermal water inputs in the Snake Creek tributary. Snake Creek accounted for only 20% of the flow to the Provo River but increased the As concentrations ~four-fold. Diffuse groundwater inputs, including thermal water, along the Provo River also contributed a measureable increase in solute concentrations. Mixing calculations indicate that groundwater contributed up to 10% of the total streamflow to the middle Provo River, causing an increase in thermal groundwater-derived trace element concentrations. In addition to natural groundwater inputs, water quality was impacted by anthropogenic trace and major element inputs from surface water tributaries. Nitrate, Ba, and V concentrations increased substantially downstream of agricultural/urban inputs. Specific conductance data showed that tributaries added solutes to the Provo River during runoff events, likely from the washoff of road salts. With evidence of both natural and anthropogenic inputs of trace and major elements to the middle Provo River, our study has implications for understanding water quality in complex coupled human–natural systems and demonstrates the influence of thermal groundwater inputs on water quality where such systems discharge.