Shallow Groundwater Zone Research Articles

Field investigation of the transport and attenuation of fugitive methane in shallow groundwater around an oil and gas well with gas migration

‘Fugitive’ or ‘stray’ gas migration from deeper formations due to well bore integrity failure has prompted concern regarding environmental impacts. Unintended methane (CH4) migration can increase greenhouse gas emissions and affect groundwater quality in the critical zone. Although the CH4 transport in shallow aquifers has been investigated at experimental injection sites, no intensive groundwater studies have been published around an oil and gas well that has been leaking for a significant period of time. In this field study, groundwater samples were collected from sixteen groundwater monitoring wells (1.25 m below ground surface) installed around a suspended oil and gas well with decadal scale gas migration (estimated ~0.2 m3/day). Stray CH4 distribution and preferential pathways in the shallow groundwater zone were evaluated though high-resolution profiling of equivalent concentrations of hydrocarbon gases (C1-C6; >85 % CH4 at the study site) and bulk formation electrical conductivity to 6.0 m below ground surface. The highest dissolved CH4 concentration (0.074 mmol/L or 1.18 mg/L) in groundwater (1.25 m bgs) was observed immediately downgradient (1.25 m) of the oil and gas well head. Similarly, high-resolution profiling data also revealed the occurrence of relatively high CH4 concentrations in shallow groundwater along the groundwater flow direction and below fine-grained layers up to 10 m distance from the well head. Microbial DNA analysis from groundwater showed significant community shifts, with the highest relative abundance and diversity of methanotrophs observed in the vicinity of the oil and gas well. This study supports findings from experimental injection and laboratory studies, which also found that significant CH4 transport i) dominantly occurs in the groundwater flow direction, and ii) laterally as free phase below fine-grained layers. The occurrence of CH4 concentrations below saturation after more than two decades of gas migration suggests limited impacts have occurred in the shallow subsurface investigated.

INVESTIGATION OF SUBSURFACE GROUNDWATER USING THE VES METHOD AROUND DENDAM LAKE, BENGKULU CITY, INDONESIA

Concerns have been raised over the availability of water in the Dendam Lake neighborhood of Bengkulu City and the areas surrounding it. The location is characterized by a number of unsuccessful water wells. These issues are the direct and indirect results of the geological formations that are present in the region, specifically Aluvium (which consists of boulders, clasts, sand, silt, mud, and clay). A MAE X-612 EM resistivity meter was utilized in order to get a total of twenty Vertical Electric Sounding (VES) measurement points. Using the software programs Progress and Surfer, the data that was gathered was plotted on a two-log graph. The geoelectric layer and layer parameter maps that were made were prepared with the use of quantitative and qualitative interpretations. It was determined that there were between three and five geoelectric layers. It has been demonstrated that the resistivity of the subsurface shallow groundwater zone ranges from 10 to 20 Ωm at a depth of 4 to 15 meters. According to the findings of this research project, employing the VES geo-electrical exploration approach is likely capable of effectively outlining the subsurface and shallow groundwater. In addition, there is no information available regarding the precise depth of the groundwater table in the region. On the other hand, the underground groundwater at this location has a depth that is shallower than the norm.

Review: Groundwater research in the Ethiopian Rift Valley Lakes region

Despite its proximity to many research institutions in the country and prevailing environmental and water security challenges, water resources research in the Central Rift Valley of Ethiopia has few decades of history. Research undertaken so far, mainly focus on the lakes' environment and anthropogenic activities in their proximities. Worsening deterioration of the water resources and environmental conditions; and the need to address overlooked but determinant natural and anthropogenic processes spurred a critical review of what has been done so far. This work provides an overview of the history of water research in the central rift valley and tries to reveal research gaps related to surface water-groundwater interaction, water quality, and changing trends in the hydro ecosystem and possible causes. Apart from this, articles dealing with the geological and structural setup of the central rift valley were systematically reviewed to show their control over the hydrologic system. The review work has revealed that although the current state of the central rift valley is a product of anthropogenic and geogenic processes; which are happening within the sub-basin and its adjacent basins that need to be addressed at a higher thematic, spatial, and temporal scopes, there is gap in reviewed research, to address the issue at this level. The forefront environmental challenges and the need for quick fixes, lack of data, and funding are found to be some of the reasons to limit the scope of research activities, mainly to shallow groundwater zones and surface hydrological processes around the lakes. This approach has hindered seeing the bigger picture and resulted in ineffective environmental and natural resources restoration measures and policy decisions.

An integrated approach for the identification of potential shallow groundwater zones in west-central Saudi Arabia

This study aimed to identify groundwater resources at Harrat Khaybar to meet the increasing demand for groundwater for drinking and other purposes, including agricultural and industrial activities. Aquifers were assigned based on integrated electrical resistivity tomography (ERT) and hydrogeological data. Twenty-three ERT profiles ranging 590–890 m long distributed across Harrat Khaybar were acquired. The ERT data were processed to obtain 2D subsurface geoelectrical sections that indicated a variable depth for groundwater as 6–280 m. To investigate water quality, 69 groundwater samples were collected from intensively pumped wells distributed throughout the Harrat. The depth to groundwater measured in these wells ranged 1.5–80 m. It was concluded that Harrat Khaybar is composed of basaltic lava flows underlain by alluvial deposits, which represent two significant water-bearing formations: sub-basaltic alluvial deposits and basalt flows. The depth to the groundwater abruptly deepened from 1.5 to 80 m in the northeast and southwest directions, which could be due to groundwater-bearing formations dipping in these directions or northwest-orientated tension faults. The groundwater quality was assessed in terms of total dissolved solids (TDS), which ranged from 225 to 8340 mg/l, and a TDS-drilling zone encountered in the western part of the Harrat, the following similar most likely resulted from groundwater mineralization. The concentration of TDS generally decreased toward the east in the groundwater flow direction. The pH (6.5–8.1) of the samples indicated that the groundwater pH meets quality standards for domestic and irrigation purposes. It is concluded that, the application of both methods will be of great benefit to identify the groundwater sources and its quality as well.

An approximate analytical solution of depth to water table driven by periodical precipitation and evapotranspiration in shallow groundwater zones

Depth to water table (DWT) for an unconfined aquifer affects a wide range of hydrological, ecological, and agricultural problems such as soil salinization, plant type, and crop output. In a world without human impacts, DWT in shallow groundwater zones is mainly driven by evapotranspiration and precipitation. In the real-world however, human impacts on groundwater hydrological processes are ubiquitous, which hinders one to fully understand the natural groundwater hydrological rule. The purpose of this study is to derive an analytical solution of precipitation-evapotranspiration driven DWT irrespective of the human impacts. Through pretreatment of practical data to eliminate human impacts, we show that the proposed method is efficient in dealing with the real-world problems. Specifically, we establish a mathematical model describing impacts of periodic evapotranspiration and precipitation on DWT. An innovative quasi-analytical solution using the Fourier series is developed and analyzed in detail based on the model. This study indicates that the amplitude of DWT is correlated linearly with the amplitude of precipitation and quadratically with the amplitude of potential evapotranspiration. The phase position of DWT is subject to the phase lag between the precipitation and potential evapotranspiration. In arid regions, the evapotranspiration has a greater influence on the phase of DWT than the precipitation. This quasi-analytical solution is limited for a shallow groundwater situation in which DWT is less than the extinction depth of groundwater evapotranspiration.

Understanding of subsurface conditions controlling flow liquefaction occurrence during the 2018 Palu earthquake based on resistivity profiles

The 7.4 Mw earthquake on 28th September 2018 in Palu City triggered a flow liquefaction phenomenon in the Balaroa and Petobo areas, contributing to significant casualties and building damage. This paper presents the results of a liquefaction study to map subsurface conditions in these areas using the multi-electrode resistivity method with the dipole-dipole configuration. The objective of this study is to understand factors controlling the flow liquefaction phenomenon. Based on the interpretation of 2-D resistivity images, the liquefied soil layers are characterized by lower resistivity values than the non-liquified layers. These contrasts of resistivity values form a gently sloping boundary between the liquefied and non-liquefied soil layers. The resistivity image perpendicular to the flow direction indicates the presence of a subsurface basinal morphology in the Balaroa area, suggesting that a shallow groundwater zone is present within the liquefiable soil layer. Thus, the subsurface topographical condition is the main governing factor of flow liquefaction phenomena during the 2018 Palu earthquake.

Methanogenesis and secondary processes in a fractured claystone aquifer influenced by a former landfill site: Implications for the interpretation of dissolved gases

Similar to marine and/or freshwater sediments, dissolved methane and longer chained hydrocarbons are ubiquitous in all natural groundwater environments including highly productive porous aquifers. The isotopic and chemical characteristics of dissolved gases in groundwater related to municipal landfills commonly differ from other non-polluted environments. In the survey presented in this study we investigated the geochemical and isotopic variations of dissolved hydrocarbons in a unique study area with different well defined ecological micro-habitats influenced by a former landfill for industrial and special waste. The dissolved methane concentrations have generally remained stable over the last decade but vary by several orders of magnitude in different zones of the area. The dominant methanogenic pathways can be clearly distinguished in the different ecosystems. Methyl-type fermentation methanogenesis prevails in the shallow groundwater zone (5–15 m depth) in- and outside the landfill. Anaerobic methane oxidation coupled to sulfate reduction occurs in the shallow downstream area within a former leachate plume. At larger depths CO2 reduction is the predominant methane formation pathway except within the physically isolated landfill site. The high ethane concentrations in a small part of the study area are potentially related to the degradation of volatile organic compounds. The background ethane concentrations are higher in the deeper groundwater zones, indicating the coupling to the CO2 reduction methanogenic pathway. The propane concentrations in most wells are below the detection limit, except within the waste site.Secondary methane oxidation, which frequently occurs in the groundwater in all environments can alter the gas composition mimicing the thermogenic gas signature in the standard Bernard-diagram. The isotopic fractionation between co-existing methane and formation water and carbon dioxide, respectively, differs from that suggested in standard interpretational diagrams (e.g. Bernard-Diagram and Whiticar-plots). Thus, the standard evaluation based on methane hydrogen and carbon isotopes is also susceptible to an erroneous interpretation as non-biogenic dissolved gases in other groundwater environments.

Hydrogeophysical Investigation for Groundwater Resources from Electrical Resistivity Tomography and Self-Potential Data in the Méiganga Area, Adamawa, Cameroon

Exploration and production of groundwater, a vital and precious resource, is a challenging task in hard rock, which exhibits inherent heterogeneity. A geophysical survey was conducted in Méiganga, Mbéré department, in the Adamawa region, Cameroon. High-resolution electrical resistivity tomography (ERT) and self-potential (SP) dataset were collected in a gneissic terrain to solve the groundwater problem as people are facing acute shortage of drinking water in the study area. The analysis and interpretations based on resistivity models revealed substantial resistivity contrast between the altered gneiss that might contain water and massive gneiss and delineated five deeper groundwater prospects zones located at Yelwa, Ngoa-Ekélé, Sabongari, Ngassiri, and Gbakoungué, respectively. Nevertheless shallow groundwater zones (<13 m) are located in the northern part of the study area at high elevation while best prospect and productive groundwater zones lying between 20 and 25 m depth are at low elevation in the southern part. On the other hand, analysis of SP negative peaks along with groundwater head and groundwater vector maps revealed areas of recharge and discharge across the study area. The discharge areas serve as groundwater collection center and are good groundwater potential zones. In addition these maps revealed that groundwater flow pattern shows inward flow from the flanks to center and south central parts of the study area.

Distribution and mass of groundwater orthophosphorus in an agricultural watershed

Orthophosphorus (OP) is the form of dissolved inorganic P that is commonly measured in groundwater studies, but the spatial distribution of groundwater OP across a watershed has rarely been assessed. In this study, we characterized spatial patterns of groundwater OP concentrations and loading rates within the 5218ha Walnut Creek watershed (Iowa) over a two-year period. Using a network of 24 shallow (<6m) monitoring wells established across watershed, OP concentrations ranged from <0.01 to 0.58mg/l in all samples (n=147) and averaged 0.084±0.107mg/l. Groundwater OP concentrations were higher in floodplains and OP mass loading rates were approximately three times higher than in uplands. We estimated that approximately 1231kg of OP is present in floodplain groundwater and 2869kg is present in upland groundwater within the shallow groundwater zone (0–5m depth). Assuming no new inputs of OP to shallow groundwater, we estimated it would take approximately eight years to flush out existing OP mass present in the system. Results suggest that conservation practices focused on reducing OP loading rates in floodplain areas may have a disproportionately large water quality benefit compared to upland areas.

Mobility of copper and zinc in near-surface groundwater as a function of the hypergenic zone lithology at the Kampinos National Park (Central Poland)

In the paper, we assess the mobility of copper and zinc in two sections, Granica and Kampinos, of the glacial and post-glacial sediments in the Kampinos National Park (Poland) and its buffer zone. The Granica section is practically devoid of carbonates and clay fraction, and the environment is markedly acidic. The other section, Kampinos, is characterized by a considerable content of carbonates (up to 20%) and clay fraction (up to 34%) with high sorption potential. The bonds of Zn2+ and Cu2+ cations with the solid phase occur mainly through adsorption and formation of salts, and through creation of chelated and complex connections. We found that, while sorption of Cu2+ within the shallow groundwater zone under typical natural environment equilibrium conditions depends mainly on the content of the sorbing solid phase (i.e. organic matter and metal oxides and hydroxides), sorption of Zn2+ within the shallow groundwater zone depends mainly on the fluctuating acidity and ionic strength of the groundwater, and to a lesser extent on the content and nature of the solid phase. The sorption process within the Kampinos section occurred predominantly on to three sorption media: clay fraction, organic matter and amorphous forms of Fe, Al and Mn. In the Granica section, because of the absence of clay fraction, sorption of the two metals occurred mainly with the organic matter and amorphous forms of Fe, Al and Mn. In addition, the sorption potential of the two metals in this section was low, because of the acidic pH of the environment, i.e. high availability of exchangeable hydrogen in the sorption complex. The concentrations of copper and zinc in the soil (So) were examined using both: the Lindsay and Norvell’s and aqua regia methods. The methods of variable extraction capacity yielded very similar results for copper. In addition, the study found that the amounts of extracted zinc depended on the type of solution. The highest concentrations of zinc and copper were observed using the extraction with aqua regia. The results from determining zinc and copper concentrations (So) in the soil and rocks and the results of hydrochemical modelling show that the mobility of these two elements is highly limited by sorption processes; moreover, zinc is less effectively adsorbed than copper in these sections.

Shift of annual water balance in the Budyko space for catchments with groundwater-dependent evapotranspiration

Abstract. The Budyko framework represents the general relationship between the evapotranspiration ratio (F) and the aridity index (φ) for the mean annual steady-state water balance at the catchment scale. It is interesting to investigate whether this standard F − φ space can also be applied to capture the shift of annual water balance in catchments with varying dryness. Previous studies have made significant progress in incorporating the storage effect into the Budyko framework for the non-steady conditions, whereas the role of groundwater-dependent evapotranspiration was not investigated. This study investigates how groundwater-dependent evapotranspiration causes the shift of the annual water balance in the standard Budyko space. A widely used monthly hydrological model, the ABCD model, is modified to incorporate groundwater-dependent evapotranspiration into the zone with a shallow water table and delayed groundwater recharge into the zone with a deep water table. This model is applied in six catchments in the Erdos Plateau, China, to estimate the actual annual evapotranspiration. Results show that the variations in the annual F value with the aridity index do not satisfy the standard Budyko formulas. The shift of the annual water balance in the standard Budyko space is a combination of the Budyko-type response in the deep groundwater zone and the quasi-energy limited condition in the shallow groundwater zone. Excess evapotranspiration (F &gt; 1) could occur in dry years, which is contributed by the significant supply of groundwater for evapotranspiration. Use of groundwater for irrigation can increase the frequency of the F &gt; 1 cases.

Metal enrichment in the groundwater of the arid environment in South Africa

This paper presents the enrichment tendency and spatial distribution of metals in the groundwater which is pumped out from the granitic aquifers in South Africa. Groundwater is the sole source of water supply for the local community in the study area (Namaqualand), and hence, it was necessary to understand the controlling geochemical processes and interrelationship of metals in the groundwater. The geochemical association of metals has been assessed based on the geostatistical methods. The results show that geochemical processes such as oxidation, leaching, and evaporation besides water–rock interaction are very important in controlling metal enrichment in the groundwater from highly mineralized rocks. The metal enrichment index for selected toxic metals in groundwater increases in the order of Cd > U > Cr > Pb. The observed enrichment trend could be considered as a result of mineralization of basement rocks which is facilitated by active geochemical process in the arid environment. The lack of aquifer flushing due to negligible recharge helps the metals to concentrate at shallow groundwater zones supported by severe evaporation process.

Study on feasibility of drip irrigation in shallow ground water zones of eastern India

Although the effectiveness and economics of drip irrigation systems are well established in areas having undulating topography and water scarcity, its economics in plain areas bestowed with good quality ground water at shallow depths are not yet studied. Due to this, there is confusion among farmers as well as planners regarding the guidelines for selection and propagation of this system in such areas. This study attempts to evolve guidelines for deciding the economic threshold value of fixed cost of drip irrigation system for sugarcane crop. Nomographs have been developed to find the threshold value of economic fixed cost of drip irrigation system (exclusive of pumping unit) for a combination of variables, depth of water table, yield level under check basin irrigation, expected yield gain with adoption of drip system, irrigation requirement with check basin, and electricity charges. The perusal of nomographs shows that the factors influencing the economics of drip irrigation in order of importance are yield gain ratio, electricity charges, irrigation requirement and depth of ground water.

Compiling the map of shallow-buried palaeochannels on the North China Plain

The 4 methods used in compiling the shallow-buried palaeochannel maps are as follows: geohydrologic, electrical-resistivity, areo electro-magnetism and drilling methods. On the North China Plain, the drilling method was used mainly accompanied by the geohydrologic and the electrical-resistivity methods. Before compiling the map, detailed studies of typical palaeochannels were undertaken of palaeochannel sediments, their ages and relation to the storage of shallow groundwater quantity and quality. This provides the theoretical foundation for compiling the map of a large area and for dividing the palaeochannels. Then, data from drilling samples were collected and each drilling hole divided into zones 1 and 2 and marked on the base map. The boundaries of zones 1 and 2 were drawn on the base map. From the map of the shallow-buried palaeochannels, we can conclude that the shallow-buried palaeochannel zones and shallow groundwater zones are identical with the sand bodies of stage I, III and IV palaeochannels.

Fracture effects in the shallow groundwater zone in weathered Sarnia-area clay : D'Astous, A Y; Ruland, W W; Bruce, J R G; Cherry, J A; Gillham, R W Can Geotech JV26, N1, Feb 1989, P43–56

Fracture effects in the shallow groundwater zone in weathered Sarnia-area clay : D'Astous, A Y; Ruland, W W; Bruce, J R G; Cherry, J A; Gillham, R W Can Geotech JV26, N1, Feb 1989, P43–56

Natural radioelement solution by circulating groundwaters in the Stripa granite

Natural radioelement concentrations in groundwaters from the fracture flow regime in the Stripa granite were monitored during the period 1977–1986. Uranium concentrations from depths of 80 m or less and in minewaters from above the hydraulic sink produced by the deepest mining level (410 m), range from 10 to 90 ppb. 234U 238U activity ratios are characteristic of chemical leaching processes at the shallower depths whilst at about 350 m groundwaters occur which have an additional component of 234U by alpha-recoil processes. At greater depths, U concentrations are indicative of the changed redox conditions and are subject to activity ratio enhancement by the alpha-recoil process. Groundwater residence times may be estimated from the rate of such recoil solution of 234U in zones where chemical leaching of U is not active. The 222Rn contents of the minewaters are extremely high and can be related to the 222Rn flux from fracture surfaces and the fracture width. The fracture apertures range from 10 mm for the shallow groundwater zone ( 222Rn contents < 400 Bq kg −1) to 20 >μm for water from the deepest borehole section ( 222Rn content 100 kBq kg −1). The activity ratios 234Th 238U , 210Pb 222Rn and 210Po 222Rn have been used to estimate residence times for Th, Pb and Po isotopes. Solution of Ra is controlled by chemical rather than alpha-recoil processes. The 226Ra content of the groundwater increases with salinity and is well correlated with groundwater Ca content. The 228Ra 226Ra activity ratios in the groundwaters suggest that water/rock interaction occurs predominantly with the average granitic matrix rather than with highly uraniferous fracture mineralisation. The behaviour of Ra, Th and Po isotopes may be used to assess that of homologous fission product and actinide constituents of nuclear wastes in groundwaters.

Fracture effects in the shallow groundwater zone in weathered Sarnia-area clay

The Sarnia area of southwestern Ontario is underlain by thick deposits of clay-rich glacial till. From ground surface to between 4 and 6 m depth the clay till is fractured, oxidized, and penetrated by root holes. The water table fluctuates seasonally between the ground surface and the bottom of the weathered zone. Water-level response tests in conventional piezometers installed in augered holes in the weathered zone typically provide very low values of hydraulic conductivity (10−7–10−9 cm/s), which are similar to values obtained from the deeper unweathered and unfractured till. The augering process creates a smeared clay zone along the borehole walls that significantly reduces measured hydraulic conductivity values. In this study, the weathered-zone hydraulic conductivity was measured using unconventional piezometers, a large-diameter well, and a tracer experiment between two test pits. The smeared zone was removed or reduced around these installations, and the measured weathered-zone hydraulic conductivity was 10−5–10−7 cm/s, much higher than in the deep unweathered clay. These results, together with depth profiles of groundwater tritium and monitored water-level fluctuations, indicate that the weathered zone has hydraulic conductivity governed by fractures and has significant hydrologic activity in the fractures. Key words: clay, fractures, hydraulic conductivity, permeability, piezometer, tritium, tracer experiment, weathered zone.

Utility of a computerized data base for hydrogeologic investigations, Las Vegas Valley, Nevada

Hydrogeologic study of the shallow groundwater zone in Las Vegas Valley, Nevada involved development of an extensive computerized data base consisting of water analyses and water-well logs. The data were manipulated and reduced using a variety of graphical and statistical techniques applicable to analysis of spatial and temporal changes in water quality. Stratigraphic relations, permeability/transmissivity variations, water budgets and ambient quality relative to drinking-water standards were evaluated as part of an overall EPA funded study to determine the sources and extent of groundwater contamination and develop management alternatives to minimize adverse effects. Extension of the data base to include investigations concerned with improved definition of the stratigraphic and structural makeup of the valley and to applied study of water quality, subsidence and water banking seems reasonable.

Field Evidence of Denitrification in Shallow Groundwater Flow Systems

Abstract A detailed hydrogeological study in a small agricultural watershed near Lake Erie in southwestern Ontario has provided evidence that denitrification is widespread in shallow groundwater in this area. In the groundwater zone of the unconfined sandy aquifer that underlies the study area, NO3- concentrations within two metres of the water table are commonly between 5 and 50 mg/1 (expressed as N). At greater depth, however, NO3- concentrations are generally less than 0.02 mg/1. The aquifer varies in thickness from 3 to 10 m. The transition zone between the high nitrate zone and the low nitrate zone is very thin. In the high nitrate zone, dissolved oxygen occurs at concentrations above 2 mg/1 , methane is absent, and Eh values obtained by the platinum electrode method are greater than 300 mV. In the low nitrate zone, dissolved oxygen occurs at levels less than 2 mg/1, methane is present, and Eh values are generally between 50 and 200 mV. The hydrogeologic conditions are such that it is very improbable that these trends are due to different groundwater sources. The data support the hypothesis that denitrification is the cause of the lack of NO3- in the deeper part of the aquifer. Nitrate and dissolved oxygen data from shallow groundwater zones in four other agricultural watersheds in southwestern Ontario support this conclusion.

The Origin of Chemical Variations in Groundwaters from a Small Watershed in Southwestern Ontario

A very striking and unique feature of the groundwater system within the Upper Kettle Creek watershed is the marked concentration decrease of nearly all ionic species in the direction of flow. The conceptual model proposed here attributes this decrease to selective ion filtration by the Port Stanley till. Ionic constituents generated in the shallow groundwater zone by dissolution of the porous medium either pass through the till or accumulate depending upon their relative mobilities. Other important processes that influence the composition of the groundwaters are cation exchange within the Port Stanley till, CO2 gas generation within the soil zones of recharge areas, and the upward leakage of H2S gas from deeper in the Paleozoic section.