Variability In Groundwater Flow Research Articles

The Ibadan Hydrogeophysics Research Site (IHRS)—An Observatory for Studying Hydrological Heterogeneities in A Crystalline Basement Aquifer in Southwestern Nigeria

Crystalline basement aquifers are important drinking water sources in Nigeria and several sub-Saharan African countries. However, an understanding of their local flow and transport processes and pathways is missing due to limited research. The implication has been their suboptimal management, with frequently reported dry wells and groundwater contaminations. To address this challenge, the Ibadan Hydrogeophysics Research Site was established in 2019 as the first field-scale hydrogeological research laboratory in Nigeria to advance understanding of the geologic, hydraulic, and hydrogeochemical variabilities within crystalline basement aquifers. The over 22,500 m2 research site with a 50 m × 50 m area used for active hydraulic testing is located within the University of Ibadan campus and is instrumented with four initial test wells extending through the weathered and fractured zones to a depth of 30 m each. Preliminary hydrogeological and geophysical studies focused on obtaining a conceptual model and knowledge of hydraulic heterogeneities to aid in detailed experimental and numerical studies. A combination of lithological logs and electrical resistivity revealed areas with subvertical fractures as low-resistivity zones (<200 Ωm), and a pumping test revealed a hydraulic conductivity range of 1.9 × 10−10 to 7.2 × 10−6 m/s. The drawdown–time curve shows flow from single-plane vertical fractures. The results of this study will serve as a basis for further targeted field and numerical studies for the investigation of variability in groundwater flow in complex crystalline basement aquifers. The presented field site is posed to support the adaptation and development of field methods for studying local heterogeneities within these aquifers in Nigeria.

Vertical stratification of microbial communities and isotope geochemistry tie groundwater denitrification to sampling location within a nitrate-contaminated aquifer

Nitrate pollution is a major threat to groundwater quality in agricultural areas. Natural attenuation of nitrate in contaminated aquifers is mediated by denitrifying microbial populations in anoxic environments. Vertical distribution of denitrifying microbial communities in aquifers is greatly influenced by groundwater redox conditions, local hydrogeological parameters, and seasonal variability in groundwater flow and recharge. In this study, we investigated groundwater geochemistry and the composition of bacterial and archaeal communities with increasing depth in a shallow nitrate-contaminated aquifer in British Columbia, Canada. High-resolution passive diffusion sampling was conducted to collect groundwater at 10-cm intervals from 4 to 20 m below ground surface (mbgs) in the aquifer. Geochemical analyses of major ions indicated a general shift in the groundwater chemistry below 16 mbgs including decreasing chloride concentrations that suggest two-end member mixing of shallow and deep groundwater with different chemistries. A redoxcline was further observed within a 2 m transition zone at 18–20 mbgs characterized by sharp declines in nitrate concentrations and increases in sulfate and total inorganic carbon. Excursions in δ15N-NO3− and δ18O-NO3− in the same depth interval are consistent with denitrification, and a concomitant decrease in δ34S-SO42− suggested that denitrification was coupled to sulfide or sulfur oxidation. Microbial communities within this depth interval were significantly dissimilar to those above and below, featuring putative lithotrophic denitrifying bacteria belonging to the genera Sulfurifustis, Sulfuritalea and Sulfuricella. These lineages were detected in greatest abundance at 19 mbgs while the abundances of putative heterotrophic sulfate-reducing bacteria belonging to the genus Desulfosporosinus were greatest at 20 mbgs. In addition to help distinguish denitrification from mixing-induced changes in groundwater chemistry, the above observed vertical stratification of the microbial key players connects nitrate removal to the locations of the aquifer sampled.

Variability in groundwater flow and chemistry in the Mekong River alluvial aquifer (Thailand): implications for arsenic and manganese occurrence

Arsenic (As) is widespread in alluvial aquifers along rivers draining the Himalayas but has received little attention along the Mekong River upstream of its delta. This study investigates linkages among groundwater recharge, flow, chemistry, and river stage at two sites along the Mekong River in northeast Thailand. Hydraulic head and chemistry were monitored in January and June 2014. In addition, hydraulic head and electrical conductivity were continuously logged at one site from January 2014 to March 2015. During the dry season, groundwater tends to flow toward the river, and saline water from shallow evaporites can intrude the alluvial aquifer. As stage rises, hydraulic-gradient reversals can result in groundwater-river water mixing, thereby promoting geochemical disequilibrium. Groundwater chemistry reflects silicate, carbonate, and evaporite weathering; multiple redox reactions; and spatial and temporal variability in recharge. Concentrations of As and manganese (Mn) in groundwater exceeded drinking-water guidelines by as much as an order of magnitude. Results suggest As is mobilized in groundwater by reduction of ferric (oxyhydr)oxides, consistent with the findings elsewhere in South and Southeast Asia. Upwelling of saline water can increase the solubility of (oxyhydr)oxide phases that sequester Mn and As, can complex Mn and can mobilize adsorbed As oxyanions. Conversely, influxes of oxic river water could promote (oxyhydr)oxide precipitation.

Variability in Groundwater Flow and Chemistry in the Houzhai Karst Basin, Guizhou Province, China

ABSTRACT Understanding how karst aquifers store and transmit water and contaminants is an ongoing problem in hydrogeology. Multiple flow paths and recharge heterogeneity contribute to the complexity of these systems. This study explored karst-conduit connectivity and water-chemistry variability within the Houzhai catchment in Guizhou Province, China. Artificial tracer tests were conducted during both the monsoon and dry seasons to understand temporal variability in connectivity and water velocity between karst features. Multiple flow paths through the catchment were activated during the monsoon season and partially abandoned during the dry season. Additionally, gradient reversals during monsoonal high-flow events and as a result of pumping were observed. Synoptic water samples from several karst features taken during both monsoon and dry seasons elucidated spatial and temporal variability within the catchment. Water residence time was generally longer during the dry season, and flow within the Houzhai catchment was determined to be temporally dependent. Time-series sampling at the outlet spring following a monsoonal storm event captured chemical variability and identified multiple flow paths. Overall, this study refines widely applicable methods for studying karst systems to this catchment and provides a foundation for future studies in similar settings.

Hydrologic interpretation of seasonally dynamic ambient temperature profiles in sealed bedrock boreholes

This study evaluates the utility of ambient temperature profiles collected in sealed bedrock boreholes to assess variability in groundwater flow in discretely fractured shallow bedrock environments. A conceptual model for groundwater flow and groundwater-surface water temperature conditions and their interaction in a temperate climate is developed through a statistical interpretation of time-lapse thermal deviation logs. Temperature profiles were collected in three angled and three vertical boreholes drilled to 24–32 mbgs (meters below ground surface) and temporarily sealed with an impermeable fabric liner in a fractured dolostone bedrock aquifer adjacent to and extending beneath a bedrock river to monitor seasonal hydrodynamics. Ambient borehole temperature profiles collected every 1–8 weeks over a 12 month period identified zones of hydraulic activity during periods of intra-seasonal stability without the interference of open borehole cross-connection. Signal cross-correlation and Fourier spectra analysis of thermal deviation logs provided a novel way to observe the shallow bedrock flow system’s temperature evolution due to advection along discrete fractures in response to seasonal transience, and to identify and isolate noise caused by free-convection cells within the sealed borehole. This approach represents a diagnostic tool that improves confidence in identifying depth discrete, hydraulically active fracture zones from thermal deviation data sets in a shallow, fractured sedimentary bedrock environment. Variably scaled free-convection cells were observed within the borehole water columns during the colder winter periods. Although these periods were accompanied by higher signal noise near the river/atmospheric interface, these cells led to a temporary thermal disequilibrium between the borehole water column and formation water deeper in the bedrock. These conditions increased the maximum depth of thermal detections associated with discrete groundwater flow features from 14 mbgs in the summer to 26 mbgs in the winter, thereby enhancing the understanding of shallow groundwater flow systems under the direct influence of surface water.

Variability of groundwater flow and transport processes in karst under different hydrologic conditions

Significance of hydrological variability in karst is presented, which also discusses factors inducing such variability and consequences it may cause. Groundwater flow in karst aquifers is often characterized by strong variability of flow dynamics in response to different hydrologic conditions within a short time period. Consequently, water table fluctuations are often in the order of tens of meters, differences in flow velocities between low- and high-flow conditions can reach ten or even more times. In dependence to respective hydrologic conditions groundwater flow also results in variations of flow directions, and thus in contribution of different parts of the aquifer to a particular spring. The described hydrological variability has many implications for contaminant transport, groundwater availability and vulnerability. Groundwater level rising reduces thickness of the unsaturated zone and decreases protectiveness of the overlying layers. Higher water flow velocities reduce underground retention. Due to more turbulent flow, transport and remobilization of solute and insoluble matter is more effective. During high-flow conditions there is usually more surface flow and hence more concentrated infiltration underground. Particularly in karst systems that show very high hydrologic variability, this should be considered to correctly characterize, understand or predict the aquifers’ hydrological behaviour and to prepare proper protection strategies.Keywords: karst aquifer, temporal hydrological variability, groundwater flow, transport processes, karst spring, water source protection.DOI: 10.3986/ac.v42i2.644

Patterns and variability of groundwater flow and radium activity at the coast: A case study from Waquoit Bay, Massachusetts

Radium is widely used as a natural tracer of groundwater discharge to the ocean. To better understand how radium activities vary within the groundwater systems that contribute this discharge to coastal waters, we characterized the spatial patterns of porewater activities of the four radium isotopes in the context of the groundwater flow system of Waquoit Bay, Massachusetts. Radium activities were highly heterogeneous across locations, though qualitatively consistent with inferred groundwater flowpaths and simple models of radium evolution. A comparison of radium activity measurements in discharging groundwater, porewater, and baywater indicates that saline submarine groundwater discharge is likely a mixture of water with long and short subsurface residence times. Flow is driven by forcing mechanisms acting on a range of timescales, from waves, to tides, to seasons and longer. Estimates of radium flux along transects extending offshore are disproportionate to groundwater discharge, and individual isotopes have distinct patterns with peak activity occurring at different locations. Modeled sensitivity of porewater radium activities to production rates and sorptive properties of the subsurface indicates that variability in radium activity is caused by aquifer property heterogeneity, variations in groundwater flowpaths, and hydrologic transience. These results suggest that, in many cases, the complexity of coastal groundwater flow and its influence on radium activity present a challenge for using radium activity measured in seawater to quantify groundwater inputs. However, characterization of the spatial pattern of radium activities within the groundwater system may provide valuable qualitative understanding of the subsurface flow processes that drive groundwater discharge.

Fuzzy-stochastic characterization of site uncertainty and variability in groundwater flow and contaminant transport through a heterogeneous aquifer

Site variabilities and uncertainties in data and information lead to significant spread in results of groundwater flow and contaminant transport models. A framework for hybrid propagation of random uncertainties represented by probability theory; nonrandom uncertainties represented by fuzzy set theory; and site variabilities represented by geostatistics was developed in this research. A case study was provided to explain the computational algorithm. The methodology presented here can be applied to complex environments where there are site variabilities as well as uncertainties of different kinds. The algorithm is suited when uncertainties in some variables may be best represented as fuzzy numbers whereas in others as probability distributions and both form part of the same governing equation.

Influence of nested groundwater systems on reduction–oxidation and alkalinity gradients with implications for plant nutrient availability in four New York fens

Summary This study compares hydrologic investigations of four New York fens designed to characterize spatial variability in groundwater flow (GWF) and the resulting hydrochemical patterns that influence plant nutrient availability. At the marl fen and two rich fen sites, hydrometric data and pore water samples collected from nested piezometers set along the GWF-path showed spatial patterns consistent with a mixing gradient established by the intersection of local (i.e., hill slope), relatively dilute groundwater discharge and mineral-rich groundwater discharged from a larger-scaled (i.e., intermediate) system. Distinct redox and carbonate gradients along the GWF-paths of three rich fens reflected strong interactions between groundwater supply of terminal electron acceptors and microbial processes affecting speciation of iron, nitrogen and phosphorus. Decreased nitrate concentrations, increased dissolved iron, and removal of sulfate along flowpaths indicated that redox conditions became more reduced as groundwater moved down-gradient. Nitrate removal occurred across the interface between the upland mineral soil and the organic-rich fen peat. Lack of ammonium accumulation indicated that denitrification limits N-supply to plants in down-gradient fen areas. Further down-gradient, sulfate-enriched groundwater discharge sustained sub-oxic conditions and enhanced phosphorus mobility through effects on iron–sulfur–phosphorus dynamics, alkalinity generation, and dissolution of mineral-bound P. In contrast, the poor fen was sustained by an isolated surficial groundwater system; a thick lacustrine clay aquitard limited intermediate or regional-scaled groundwater discharge. As a result, water table fluctuations were more responsive to short-term weather events and imposed the strongest influence on redox status. A redox gradient, evidenced by increasing iron and ammonium concentrations, extended downward with depth from the aerated peat surface. The dilute water supply and lack of additional internal alkalinity generation limited buffer capacity and decomposition. Overall, ammonium accumulation increased N-availability whereas reduced decomposition rates further limited P availability. Our results show that interaction of multiple GWF systems within the fens was the critical determinant of water table stability (rich fens) or fluctuation (poor fen), as well as the supply of dissolved ions that influence geochemical processes affecting plant nutrient availability. Together these factors accounted directly or indirectly for observed spatial patterns in redox and alkalinity gradients, which in turn controlled speciation of phosphorus-reactive minerals. Differences among fens in spatial patterns could be accounted for by differences in the geomorphometry of the basins within which the fens developed and the stratigraphy and hydraulic characteristics of underlying sediments. Results suggest that Fe–S dynamics, rather than carbonate precipitation, likely influences inorganic phosphorus pools and release mechanisms.

Modelling of physical and reactive processes during biodegradation of a hydrocarbon plume under transient groundwater flow conditions

Numerical experiments of non-reactive and reactive transport were carried out to quantify the influence of a seasonally varying, transient flow field on transport and natural attenuation at a hydrocarbon-contaminated field site. Different numerical schemes for solving advective transport were compared to assess their capability to model low transversal dispersivities in transient flow fields. For the field site, it is shown that vertical plume spreading is largely inhibited, particularly if sorption is taken into account. For the reactive simulations, a biodegradation reaction module for the geochemical transport model PHT3D was developed. Results of the reactive transport simulations show that under the site-specific conditions the temporal variations in groundwater flow do, to a modest extent, affect average biodegradation rates and average total (dissolved) contaminant mass in the aquifer. The model simulations demonstrate that the seasonal variability in groundwater flow only results in significantly enhanced biodegradation rates when a differential sorption of electron donor (toluene) and electron acceptor (sulfate) is assumed.

Brown Tide blooms in Long Island’s coastal waters linked to interannual variability in groundwater flow

There has been a widespread increase in the reporting of harmful and ‘nuisance’ algal blooms in the coastal ocean over the past few decades. On the global scale this is suspected to be a consequence of coastal eutrophication, however, on a case‐by‐case basis there is usually insufficient evidence to discriminate between the effects of human and natural causal factors. Intense blooms of the ‘Brown Tide’ unicellular algae (Aureococcus anophagefferens) have occurred sporadically since 1985 in coastal waters of Eastern Long Island and have devastated the local commercial scallop fishery. Analysis of an 11‐year time‐series dataset from this region indicates that bloom intensity is correlated with higher salinities and inversely correlated with the discharge of groundwater. Laboratory and field studies suggest that whereas salinity is unlikely to represent a direct physiological control on Brown Tide blooms, the addition of inorganic nitrogen tends to inhibit Brown Tide blooms. Budget calculations indicate that the inorganic nitrogen supply from groundwater is 1–2 orders of magnitude higher than any other external source of nitrogen for this ecosystem. Biweekly time series data collected in 1995 demonstrate that Brown Tide blooms utilize dissolved organic nitrogen (DON) for growth, as evidenced by a large decrease in DON parallel with an increase in cell abundance. On an interannual basis, bloom intensity was also positively correlated with mean DON concentrations. We hypothesize that bloom initiation is regulated by the relative supply of inorganic and organic nitrogen, determined to a large extent by temporal variability in groundwater flow. The 1980s and 1990s were characterized by exceptionally high and interannually variable groundwater discharge, associated with a large‐scale climate shift over the North Atlantic. This, coupled with the time‐lagged discharge of groundwater with high nitrate concentrations resulting from increased fertilizer use and population increase during the 1960s and 1970s, may have been a key factor in the initiation of Brown Tide blooms in 1985.