Urban Karst Research Articles

Temporal Variations of Water Chemistry in the Wet Season in a Typical Urban Karst Groundwater System in Southwest China.

It is important to investigate temporal variations of water chemistry for the purpose of improving water quality in karst groundwater systems. Groundwater samples were collected daily under various land uses of Guiyang. Major ions and stable carbon isotope composition of dissolved inorganic carbon (δ13CDIC) were analyzed to understand the biogeochemical processes. The water chemistry was dominated by Ca2+, Mg2+, HCO3-, and SO42-, which mainly derived from the dissolution of carbonate rocks (limestone and dolomite) and oxidation of sulfide. The groundwater was defined as of the HCO3-Ca Mg and HCO3·SO4-Ca·Mg type, according to its hydrochemical characteristics. Results suggested that hydrochemical concentrations changed quickly, in response to rainfall events. The fast response revealed that karst groundwater was easily impacted by rainfall and anthropogenic inputs according to temporal variation of water chemistry. The distribution of DIC (dissolved inorganic carbon) and δ13CDIC showed that DIC is mainly sourced from soil CO2(g) influx and carbonate dissolution. δ13CDIC and major ions ratios suggested that carbonate minerals were dissolved by H2SO4 at groundwater in wooded area, contributing an important source for DIC due to the slight enrichment of heavy δ13CDIC. More negative δ13CDIC values were observed after rainfall reflected the fact that soil CO2(g) and organic carbon oxidation influxes accounted for a large share during DIC formation. Various δ13CDIC and hydrochemical patterns were observed under various land use and human activity conditions. Meanwhile, relative high nitrate loads were found in groundwater after rainfall, suggesting high anthropogenic inputs following rainwater as having side effects on water quality. This study suggests that water chemistry and isotopic proof provide a better understanding of water quality and carbon dynamics responding to rainfall events in the karst groundwater systems.

Characteristics, Distribution, and Source Analysis of the Main Persistent Toxic Substances in Karst Groundwater at Jinan in North China

Karst water in northern China is an important water source for economic and social development in its distribution area and water source for ecological function guarantee. With the enhancement of human activities, the water quality of karst water in the north has deteriorated and the bearing capacity of economic activities and social and ecological environment has weakened. The study on the distribution characteristics and source analysis of toxic substances in typical karst springs in northern China is of great practical significance for the sustainable development of karst areas in northern China. Based on the abundant karst water samples, the concentration, detection, and spatial distribution of toxic heavy metals and typical organic substances in karst waters were analyzed, the quality was evaluated, and the source of substances was analyzed. The results show that the toxic substances in karst water in Jinan are mainly distributed in the northern part of the Jinan urban karst water subsystem and Baiquan karst water subsystem in Jinan city, and the detection rate is low in other areas. The main distribution areas of toxic substances in karst water are the northeast and southwest parts of Jinan city. In the recharge area and runoff area of the karst spring area, the contribution rate of toxic substances to the comprehensive quality evaluation of karst water is relatively low and relatively high in the drainage area of the spring area. The organic index contributes more to the comprehensive quality evaluation of karst water. The distribution of toxic substances is related to the historical and current industrial layouts, and industrial discharge or leakage is the primary source of pollutants in karst water. The concentration of toxic substances in karst water has persistence and inheritance. This study enriches the research results on the distribution and sources of toxic substances in typical karst springs in northern China and provides essential data support for scientific understanding of the changes in karst water quality and optimization of water resource management in northern China.

Application of Cross-borehole Integrated Geophysical Methods for the Detailed Investigation of Karst in Urban Metro Construction

With the significant development of China's metro construction, the development of urban underground karst poses a serious threat to related tunnel construction and public safety, with frequent occurrences of mud and water inrushes during tunnel construction and urban ground subsidence events. Because of the complex, urban, and shallow geological conditions and construction environments, conventional geophysical methods cannot meet the requirements for high-precision detection of small-scale and inhomogeneous complex geological bodies. Based on numerical simulation, herein we comprehensively applied both cross-borehole electrical resistivity tomography (ERT) and cross-borehole seismic computed tomography (CT) to urban underground karst surveys of the Hangzhou-Fuyang intercity railway. The results showed that: 1) under limited urban construction conditions, the use of advanced geophysical monitoring equipment greatly improved construction efficiency; 2) utilizing drilling geological results to calibrate the abnormal geophysical field attribute parameters (including wave velocity and resistivity) improved the accuracy of karst exploration and reduce defective geophysical multi-explanation effects; 3) applying the joint comparative explanation of both velocity and resistivity profiles can distinguish and explain karst and fracture development zones; 4) 550 pairs of velocity and resistivity profiles were obtained which revealed 258 karst cave anomalies and 5 fracture development zones which integrated detection accuracy exceeded the 1 m level. Thus, the high-precision joint cross-borehole tomography technology was shown to be useful for guiding intercity railway construction.

Hydrologic Modeling for Sustainable Water Resources Management in Urbanized Karst Areas.

The potential of karst aquifers as a drinking water resource is substantial because of their large storage capacity gained in the course of carbonate dissolution. Carbonate dissolution and consequent development of preferential paths are also the reasons for the complex behavior of these aquifers as regards surface and underground flow. Hydrological modeling is therefore of paramount importance for an adequate assessment of flow components in catchments shaped on karsts. The cross tabulation of such components with geology, soils, and land use data in Geographic Information Systems helps decision makers to set up sustainable groundwater abstractions and allocate areas for storage of quality surface water, in the context of conjunctive water resources management. In the present study, a hydrologic modeling using the JAMS J2000 software was conducted in a karst area of Jequitiba River basin located near the Sete Lagoas town in the state of Minas Gerais, Brazil. The results revealed a very high surface water component explained by urbanization of Sete Lagoas, which hampers the recharge of 7.9 hm3 yr−1 of storm water. They also exposed a very large negative difference (−8.3 hm3 yr−1) between groundwater availability (6.3 hm3 yr−1) and current groundwater abstraction from the karst aquifer (14.6 hm3 yr−1), which is in keeping with previously reported water table declines around drilled wells that can reach 48 m in old wells used for public water supply. Artificial recharge of excess surface flow is not recommended within the urban areas, given the high risk of groundwater contamination with metals and hydrocarbons potentially transported in storm water, as well as development of suffosional sinkholes as a consequence of concentrated storm flow. The surface component could however be stored in small dams in forested areas from the catchment headwaters and diverted to the urban area to complement the drinking water supply. The percolation in soil was estimated to be high in areas used for agriculture and pastures. The implementation of correct fertilizing, management, and irrigation practices are considered crucial to attenuate potential contamination of groundwater and suffosional sinkhole development in these areas.

Assessing "Urban Karst" Effects from Groundwater-Storm Sewer System Interaction in a Till Aquitard.

Storm sewer systems and their associated utility trenches may strongly influence the effects of urbanization on a groundwater system. This study was undertaken to identify the causes of district-wide basement infiltration in an aquitard system. It comprised widespread continuous monitoring of utility trench wells and dye tracing from storm sewer system exfiltration tests. The results indicate that a major effect of urbanization on shallow groundwater is related to storm sewer system exfiltration, which is marked by a characteristic pattern of head variations in the aquitard unrelated to distributed surface infiltration. The aquitard constrains flow from storm sewer system exfiltration to the utility trench, creating an urban flow path for groundwater discharge. Temporary buildup of water levels in the utility trench drives relatively high-velocity flow through the permeable sewer bedding material of the utility trench to a separate foundation drainage collector system, ultimately causing a severe "urban karst" effect that produces system surcharging and widespread basement water infiltration. The main conditions causing the "urban karst" are the large hydraulic conductivity ratio between the utility trench material and the aquitard, and the shallow depth and low gradient of the storm sewer system imposed by a very flat drainage basin.

Hydrogeological characterization and environmental effects of the deteriorating urban karst groundwater in a karst trough valley: Nanshan, SW China

The unique hydrogeology of karst makes the associated groundwater respond quickly to rainfall events and vulnerable to anthropogenic pollutions. In this study, high-frequency monitoring of spring discharge, temperature, electrical conductivity (EC) and pH, along with monthly hydrochemical and microbial analyses, was undertaken at the outlet of Laolondong karst underground river in Nanshan, southwestern China. The aim was to explore the environmental effects of the catchment’s urban area on the karst groundwater resources. The monitoring data of a tracer test and the response of discharge to rainfall events demonstrate that conduits and narrow fissures coexist in the Laolongdong karst aquifer. The EC, Na+, Cl− and SO42− values (840 μS/cm, 33.7, 38.6 and 137.2 μg/L, respectively), along with high concentrations of fecal coliform bacteria, at the outlet indicate considerable urban pollution in this area. The contaminants sulfate and nitrate showed different relationships with discharge and EC in different stages of a rainfall event. This behavior provided information about aquifer structure and the influence of transport properties. Meanwhile, the hydrological processes of groundwater flow could be modified by urbanization and result in increasing magnitude of urban floods in the underground river. In addition, sulfuric and nitric acids introduced by urbanization not only impact the karst groundwater quality, but also result in a significant perturbation to the carbon cycling system in the karst area.

Impacts of urban areas and urban growth on groundwater in the Great Lakes Basin of North America

The Great Lakes Basin (GLB) holds vast reserves of groundwater, the great majority of which eventually drains to the lakes. Urban growth significantly affects both the quality and quantity of this groundwater and thereby represents a potential threat to the long-term viability of the Great Lakes hydrologic system. Urban areas import, manufacture, store, transport, and utilise large volumes of chemicals, a proportion of which inevitably finds its way to the shallow sub-surface. In many cases, potentially polluting chemicals are applied directly to urban surfaces (e.g. as road salts, fertilizers and pesticides), are stored in the subsurface (e.g. gasoline tanks) or are released to the subsurface (e.g. septic systems). Because most of the basin's larger urban areas rely almost exclusively on lake-based supplies, very little attention is given to the accumulation of contaminants in shallow urban groundwaters and the serious risks they pose. Assessment of the problem is complicated by the widespread use of urban fill and a complex network of drains, pipes and tunnels that create “urban karst”, a shallow artificial aquifer, unique to urban settings, that exerts a major, yet often unpredictable influence on groundwater flow and contaminant transport. Management of ground water pollution, and its impact on the receiving Great Lakes, will require rigorous audits of all urban sources of contamination together with the development and calibration of groundwater flow and transport models that will enable the fate of urban pollutants to be reliably predicted even when groundwater is not used for supply.

Recharge sources and hydrochemical evolution of an urban karst aquifer, Sete Lagoas, MG, Brazil

The current water supply for the city of Sete Lagoas, Brazil, is almost entirely groundwater from the Sete Lagoas Karst Aquifer, with a small contribution from a fractured-basement rock aquifer. Characterizing the hydrogeologic processes is important for proper water resource management, avoiding contamination and other future issues. The main goal of this study is to identify active hydrogeological processes, such as possible surface–ground water interactions, sources of recharge, and the hydrochemical evolution of groundwater through the use of stable isotopes 18O and 2H and major ion data. The groundwater samples were collected from the karst aquifer (central urban area) and from the fractured aquifer (southern part of the city). Surface water samples were collected from the seven main lakes, and rainwater was sampled over a period of one year. The stable isotope results indicated the groundwater origin is directly from local precipitation, having a limited recharge period, and locally receiving surface water contributions. Groundwater quality in the central urban area may be influenced by surface water infiltration where the karst aquifer is in contact with overlying unconsolidated sediments. Some samples with nitrate concentration of 10 mg/L or higher indicates the natural composition of the groundwater has been altered by urban sewage contamination. The highest concentrations of major ions were found in the central region, where the most karstified area from the Sete Lagoas Formation is located. These data suggest a longer residence time for this water, indicating more mineralization, in accordance with the geological information.

Analysis of drip water in an urban karst cave beneath the Hungarian capital (Budapest)

Our geological heritage is increasingly threatened by anthropogenic activity. This is especially true of the Pal-volgyi Cave System beneath Budapest. It is among the 150 longest and at the same time most endangered cave systems in the world. The aims of the study were (i) to set up a monitoring system in the cave, (ii) to track the daily changes in the quality and quantity of drip water, and (iii) to determine the exposure of the cave. Monitoring was conducted at two locations in a shallow area next to a fracture zone (site name: TG) and one lying in a tectonically less disturbed, geologically more homogeneous location 20 m deeper (site YC). The data obtained in respect of 13 variables were assessed using descriptive statistics, principal component- and periodicity analyses. At first glance, it was apparent that the eight water quality parameters differed in quantity between the two sites. Furthermore, using principal component analysis it was shown that in the fractured-shallow setting, anthropogenic activity (external urban pollution, e.g. de-icing, decrease of land cover etc.) is the driving process determining water quality. At the tectonically less fractured site (YC) external influences originating above ground may be added to the natural karst-forming processes. The assessment of drip intensity and electric conductivity again highlighted the differences between the sites in terms of their reaction to precipitation. With regard to diurnal periodicity, although pH and Eh indicated a mature periodic behavior at both sites (covering 56−65 % of the total observed time), at site TG electric conductivity displayed diurnal periodicity over only 21 % of the total time, compared to 56 % at YC. All results pointed towards a conclusion that at site YC daily periodicity and water quality are much more connected to natural processes, while at site TG anthropogenic external influences suppress these. Key words: cave drip water, hydrochemistry, karst, Pal-volgyi Cave System, time series analysis, urban pollution. Analiza prenikle vode v urbani kraski jami pod madžarsko prestolnico (Budimpesta) Nasa geoloska dediscina je vse bolj ogrožena zaradi clovekovih dejavnosti. To se posebej velja za jamski sistem Pal-volgyi pod Budimpesto. Sodi med 150 najdaljsih in hkrati najbolj ogroženih jamskih sistemov na svetu. Cilji raziskave so bili (i) vzpostaviti sistem spremljanja izbranih parametrov v jami, (ii) spremljati dnevne spremembe kakovosti in kolicine prenikle vode, in (iii) dolociti izpostavljenost jame onesnaženju. Raziskava je bila izvedena na dveh lokacijah v plitvem obmocju blizu razpoklinske cone (ime vzorcnega mesta: TG) in na tektonsko manj pretrti in geolosko bolj homogeni lokaciji 20 m globlje (ime vzorcnega mesta: YC). Pridobljeni podatki 13 spremenljivk so bili obdelani s pomocjo opisne statistike, analize glavnih komponent in periodicostno analizo. Na prvi pogled je bilo ocitno, da se je osem parametrov kakovosti vode kvantitativno razlikovalo med obema mestoma. Poleg tega se je s pomocjo analize glavnih komponent pokazalo, da je mesto v plitvem pretrtem obmocju pod velikim vplivom antropogenih aktivnosti (urbano onesnaženje, npr. sredstva proti zmrzovanju, zmanjsanje pokrovnih plasti, itd.), ki vplivajo na kakovost vode. Na tektonsko manj preoblikovanem mestu (YC) se zunanji vplivis povrsja mesajo z naravnimi kraskimi procesi. Ocena intenzivnosti kapljanja in elektricne prevodnosti je ponovno izpostavila razlike med vzorcnimi mesti v smislu njunega odziva na padavine. V zvezi z dnevno periodicnostjo je vzorcno mesto TG kazalo dnevno periodicnost vec kot 21 % celotnega casa, mesto YC 56 %, ceprav sta pH in Eh izkazali zrelo periodicno obnasanje (56−65 % celotnega opazovanega casa) na obeh mestih. Vsi rezultati kažejo na sklep, da je na mestu YC dnevna periodicnost in kakovost vode veliko bolj povezana z naravnimi procesi, medtem ko jih na mestu TG antropogeni zunanji vplivi zadusijo. Kljucne besede: jamska prenikla voda, hidrokemija, kras, jamski sistem Pal-volgyi, analiza casovne vrste, urbano onesnaževanje.

Weekly variations of discharge and groundwater quality caused by intermittent water supply in an urbanized karst catchment

Summary Leaky sewerage and water distribution networks are an enormous problem throughout the world, specifically in developing countries and regions with water scarcity. Especially in many arid and semi-arid regions, intermittent water supply (IWS) is common practice to cope with water shortage. This study investigates the combined influence of urban activities, IWS and water losses on groundwater quality and discusses the implications for water management. In the city of As-Salt (Jordan), local water supply is mostly based on groundwater from the karst aquifer that underlies the city. Water is delivered to different supply zones for 24, 48 or 60 h each week with drinking water losses of around 50–60%. Fecal contamination in groundwater, mostly originating from the likewise leaky sewer system is a severe challenge for the local water supplier. In order to improve understanding of the local water cycle and contamination dynamics in the aquifer beneath the city, a down gradient spring and an observation well were chosen to identify contaminant occurrence and loads. Nitrate, Escherichia coli , spring discharge and the well water level were monitored for 2 years. Autocorrelation analyses of time series recorded during the dry season revealed weekly periodicity of spring discharge (45 ± 3.9 L s −1 ) and NO 3 -N concentrations (11.4 ± 0.8 mg L −1 ) along with weekly varying E. coli levels partly exceeding 2.420 MPN 100 mL −1 . Cross-correlation analyses demonstrate a significant and inverse correlation of nitrate and discharge variations which points to a periodic dilution of contaminated groundwater by freshwater from the leaking IWS being the principal cause of the observed fluctuations. Contaminant inputs from leaking sewers appear to be rather constant. The results reveal the distinct impact of leaking clean IWS on the local groundwater and subsequently on the local water supply and therefore demonstrate the need for action regarding the mitigation of groundwater contamination and reduction of network losses from sewer leakage. Furthermore, these investigations contribute to an improved understanding of urban water cycle systems in the Middle-East which may help water managers in the region to conserve precious resources.

Quantitative foundation stability evaluation of urban karst area: Case study of Tangshan, China

Karst is a potential geological disaster threatening urban construction. However, the existing methods for evaluating the stability of karst foundations are qualitative. This makes it difficult to propose reasonable and practical suggestions for optimal treatment measures in engineering projects. This paper presents a set of quantitative comprehensive evaluation methods to analyze the karst foundations in a residential area of Tangshan, China. Analyses of the karst formation mechanism and development law are undertaken in combination with a numerical simulation to evaluate the stability of the karst foundations of urban high-rise buildings using the results of a field research to determine the model and the parameters for the numerical calculation. The depth of influence of the foundation of an 8-storey building is less than 30m. Thus, treatment of the building foundation is not required if there is no karst in the range of 30m under the surface. However, foundation treatment in the karst zone up to 7m is required for a 26-storey building with a depth of influence of about 60m in order to meet the requirements of foundation stability.The employment of a combined approach method is proposed, therefore, as it provides a systematic and practical method for the challenge of optimizing the foundation treatment method for urban high-rise structures in karst areas.

Hydrological Impacts of Urbanization and Urban Roof Water Harvesting in Water-limited Catchments: A Review

Roof water harvesting is a potential source of water for domestic and livelihood uses in water-scarce urban areas of the world such as sub-Saharan Africa (SSA). However, little is known about the hydrological impacts of incorporating roof water harvesting on on-site and downstream hydrology of urbanized catchments. Therefore, the current review investigates the effects of urbanization and urban roof water harvesting on hydrological processes, rainfall-runoff relationships, groundwater recharge and water contamination, and highlights future research directions. The review showed that the urban heat island effect increases the frequency and magnitude of convective storms. The high proportion and connectivity of impervious surfaces reduce infiltration, thereby increasing the runoff coefficient and Hortonian runoff. Urbanization reduces the minimum threshold rainfall for runoff generation, resulting in multi-peak hydrographs reflecting the contribution of both pervious and impervious surfaces. Urban roof water harvesting increases catchment lag time, but reduces downstream peak and total discharge, baseflow and flow velocity. Utility trenches, tunnels and buried structures form a complex network resembling a shallow urban karst system, which provides preferential flow pathways for groundwater recharge by imported water via leakages. Contrary to the widely held notion that urbanization reduces groundwater recharge by increasing impervious surfaces, empirical evidence shows significant urban-enhanced recharge in water-limited urban catchments. However, we contend that excessive groundwater abstraction for multiple uses in water-scarce regions offsets the urban-enhanced recharge, resulting in groundwater depletion. Due to the overriding collective effects of reduced soil moisture and vegetation cover on evapotranspiration in water-limited environments, we conclude that urbanization lowers evapotranspiration. Urban roof water harvesting short-cuts the urban water cycle, thereby minimizing the risk of runoff contamination that could occur during its extended flow over contaminated land surfaces. Contaminated sources of recharge, such as wastewater leakages coupled with the urban karst system, promote groundwater pollution. Overall, urban roof water harvesting imparts additional complexity to urban catchments, and has potentially adverse effects on ecohydrology. Understanding these impacts is critical for planning, designing and operation of urban roof water harvesting systems. Future research may provide a comprehensive understanding of these impacts by combining hydrological measurements and process modelling in urbanized catchments incorporating roof water harvesting.

Determining a relationship between a newly forming sinkhole and a former dry stream using electric resistivity tomography and very low-frequency electromagnetics in an urban karst setting

Sinkhole formation is an ongoing hazard for urban karst settings, compromising construction foundations, and accelerating groundwater pollution. In Springfield, Missouri, the uppermost exposed unit is the Burlington limestone, a carbonate bedrock which is susceptible to karst formation, including caves and sinkholes. This study site concerns a detention basin near a major highway interchange, in which a new sinkhole is forming. Electric resistivity, very low-frequency electromagnetics, and dye-tracing surveys were performed at this sinkhole to determine its subsurface extent and any relation to surface features. The low resistivity associated with the sinkhole and thin soil contrasts with the high resistivity of the underlying Burlington limestone. This contrast highlights a curvilinear trough of thicker soil, which correlates with a former intermittent stream and represents a stream channel, which was graded and filled during construction of the highway and basin. The newly formed sinkhole lies entirely within this former stream channel. This relationship between the sinkhole and stream provides useful insights into the effects of urbanization on sinkhole formation. Knowledge of these relationships allows urban managers to better understand the risk this sinkhole poses to major highways and public buildings in the area.

Acute Toxicity of Copper Sulfate and Potassium Dichromate on Stygobiont Proasellus: General Aspects of Groundwater Ecotoxicology and Future Perspectives

Karst systems harbor large groundwater resources for human consumption and represent an important habitat for rare and unprotected specialized animals, the so-called stygofauna. Due to the highly adapted features towards underground life, together with the geographic isolation provided by the subterranean aquifers, groundwater-dwelling animals may lose the ability to face sudden changes on their ecosystems, and therefore the risk of extinction is remarkably high. A little is known about their sensitiveness, especially linked to contamination pressure in urbanized karst areas. Understanding the impact of contaminants on stygofauna is important for setting groundwater environmental quality and management of karst systems. We have investigated acute toxicity responses in two endemic stygobiont species of the peri-Mediterranean genus Proasellus from two different karst areas and in freshwater standard species Daphnia magna exposed to two contaminants (copper sulfate; potassium dichromate). Groundwater from both sites was characterized in order to depict possible responses resulting from the long-term exposition of organisms to contaminants. Stygobiont Proasellus spp. were remarkably more tolerant than the epigean D. magna. The less groundwater-adapted revealed to be more tolerant to acute exposure to both toxics, suggesting that the degree of adaptation to groundwater life can influence the acute response of Proasellus spp. to pollutants, and that the tolerance to wide environmental conditions could be a key factor in groundwater colonization. This study highlights the worldwide need to use local specimens to infer the effects of pollution in their corresponding karst systems, which is important to define specific environmental quality thresholds for groundwater ecosystems that will certainly contribute for its protection.

Groundwater chloride response in the Highland Creek watershed due to road salt application: A re-assessment after 20 years

Summary Chloride from road salt enters streams primarily through surface runoff and groundwater discharge. Monitoring of dry-weather flow chloride concentrations in the Highland Creek watershed of the eastern Greater Toronto Area indicates the presence of a previously unrecognised, dual porosity aquifer system whereby preferential flow associated with “urban karst” exerts a significant influence on baseflow chloride concentrations early in the year. A chloride mass balance undertaken annually over four successive salting seasons suggests that as much as 40% of the chloride applied as road salt enters the shallow aquifer resulting in a net accumulation of chloride and a gradual increase in mean baseflow chloride concentrations. Assuming current road salt application rates are continued, late summer baseflow chloride concentrations will reach around 505 mg/L, almost double present levels. Elevated chloride concentrations can affect the potability of water (the Canadian aesthetic drinking water quality guideline for chloride is 250 mg/L) and can also be toxic to aquatic organisms (CCME aquatic chronic toxicity guideline is 208 mg/L). Meeting these guidelines would require that the release of salt-laden runoff to the subsurface be reduced by over 50%.

시추공 전기비저항 토모그래피를 이용한 비저항 영상화: 과잉취수에 의한 석회암 지반침하 지역 사례

석회암 공동이 발달한 도심지역에서 지하수위 하강에 수반되어 발생한 함몰형 지반침하의 원인 규명 및 공동의 분포 특성 파악을 위하여 시추공을 이용한 전기비저항 토모그래피탐사를 실시하였다. 이때 지하수 수리지질 특성을 파악하기 위하여 시추코아의 비저항 측정, 지하수위 측정 및 수리전도도 해석을 병행하였다. 연구 지역에서의 완만한 지하수위 분포 특성과 0.8-<TEX>$9.3{\times}10^{-4}\;cm/s$</TEX> 범위의 수리전도도 분포로 부터 연구지역의 수리지질 특성은 불균질성이 크지 않은 것으로 나타났다. 시추코아를 이용한 전기비저항 측정 결과 연구지역의 석회암은 파쇄가 많은 경우, 변질이 심한 경우 및 신선한 경우로 나눠지며, 전기비저항은 각각 103-161, 218-277 및 597-662 ohm-m의 범위로 나타났다. 시추결과 점토로 충전된 석회암 공동 지점은 토모그래피 탐사자료의 역산 결과 50 ohm-m 이하의 낮은 비저항으로 나타났으며, 각 시추공 간 비저항 영상 단면으로부터 연구지역 전체적으로 지표 하부 심도 약 10-20 m 구간까지 파쇄대 또는 석회암 공동 구간이 분포하는 것으로 나타났다. 또한 석회암 공동의 직경은 약 4-6 m 규모로, 대부분 점토질로 충전된 것으로 판단된다. Electrical resistivity tomography surveys using boreholes were applied to reveal the cause of a catastrophic land subsidence accompanied by the excessive groundwater withdrawal in urban karst area and to map the connectivity of disseminated cavities over the study area. In order to understand the hydrogeological characteristics, resistivity using exsitu core samples, groundwater level for five boreholes, and hydraulic conductivity using slug test were measured. The hydraulic conductivity variation ranging from 0.8 to <TEX>$9.3{\times}10^{-4}\;cm/s$</TEX> for five boreholes and a gentle slope of groundwater level indicated that there is no significant characteristics of hydraulic heterogeneity. Core samples of the lime-silicated rock were classified as three groups including cracked, weathered, and fresh and measured the resistivity values ranged from 103 to 161, 218 to 277, and 597 to 662 ohm-m, respectively. Drilling results that showed the cavity filled with clay materials and tomogram for this region indicated resistivity value lower than 50 ohm-m. From the inverted resistivity results for each section with five boreholes, cavity and fractured layer were distributed along the depth between 10 and 20 m overall area and cavities ranging from 4 to 6 m filled with clay materials.

Potential flood hazard due to urban expansion in the karst mountainous region of North China

Since China’s economic reform in 1978, the cities of China have experienced rapid expansion and urbanization, thereby profoundly transforming the spatial pattern of urban land use in the karst regions, particularly in the urban mountainous area within city, and urbanization has had and continues to have a negative impact on urban mountain area in cities of China. With the rapid development of urbanization and civilization in these regions, environmental degradation has been increasingly serious, especially in the urban karst mountain area, thereby resulting in the destruction and breakage of urban hills that induce serious natural hazard, i.e., flood hazard.

Characterization of the hydrogeology of the sacred Gihon Spring, Jerusalem: a deteriorating urban karst spring

The Gihon Spring, Jerusalem, is important for the major monotheistic religions. Its hydrogeology and hydrochemistry is studied here in order to understand urbanization effects on karst groundwater resources, and promote better water management. High-resolution monitoring of the spring discharge, temperature and electrical conductivity, was performed, together with chemical and bacterial analysis. All these demonstrate a rapid response of the spring to rainfall events and human impact. A complex karst system is inferred, including conduit flow, fissure flow and diffuse flow. Electrical conductivity, Na+ and K+ values (2.0 mS/cm, 130 and 50 mg/l respectively) are very high compared to other nearby springs located at the town margins (0.6 mS/cm, 15 and <1 mg/l respectively), indicating considerable urban pollution in the Gihon area. The previously cited pulsating nature of the spring was not detected during the present high-resolution monitoring. This phenomenon may have ceased due to additional water sources from urban leakage and irrigation feeding the spring. The urbanization of the recharge catchment thus affects the spring water dramatically, both chemically and hydrologically. Appropriate measures should therefore be undertaken to protect the Gihon Spring and other karst aquifers threatened by rapid urbanization.

Integrating field and numerical modeling methods for applied urban karst hydrogeology

Abstract. Infrastructures constructed on unstable geologic formations are prone to subsidence. Data have been collected in the context of an upgrading project for a highway located beside a river dam constructed on gypsum-bearing formations. Surface water infiltrates upstream of the dam, circulates through the gravel deposits and into the weathered bedrock around and beneath the dam, and exfiltrates downstream into the river. As a result, an extended weathering zone within the bedrock and preferential flow paths within voids and conduits developed as part of a rapidly evolving karst system. Enhanced karstification in the soluble units of the gypsum-bearing formations resulted in subsidence of the dam and the highway. Since 2006, changes in the groundwater flow regime have been investigated by different methods that allowed the evaluation of the long-term performance of the infrastructures. Geological (outcrops, lithostratigraphic information from boreholes), hydrometrical (extensive groundwater monitoring, dye tracer tests) and hydrogeophysical (Electrical Resistivity Tomography, ERT) data were integrated into high-resolution 3-D hydrogeological and 2-D karst evolution models. The applied methods are validated and the sensitivity of relevant parameters governing the processes determined. It could be demonstrated that the applied methods for karst aquifer characterization complement each other. Short-term impacts and long-term developments on system-dynamics and the flow regime could be evaluated. This includes the description of the transient character of the flow regime during and after episodic flood events (surface-groundwater interaction, conduit and diffuse model outflow) as well as the evaluation of time scales for karst evolution. Results allow the optimization of investigation methods for similar subsidence problems, ranging from general measurements and monitoring technologies to tools with predictive utility.

Effects of restricted recharge in an urban karst system

Urban karst systems are typically considered more vulnerable to contamination and excess storm discharge because of potential source areas, increased sediment loading, and focusing of water from impervious surfaces. However, urban hydrology can lead to unexpected patterns, such as pirating of recharge into man-made storm systems. Valley Creek Basin in southeastern Pennsylvania, presents such an urban karst system. Four springs were monitored for suspended sediment, water chemistry, and storm response for an 18-month period. The baseflow suspended sediment concentrations were low, less than 4.0 mg/l. Furthermore, trace metal analysis of baseflow water samples and spring mouth sediment showed only low concentrations. The response to storms within the system was rapid, on the order of 1–3 h. The maximum water stage increases at the urban springs were typically less than 15 cm, with springs from more commercialized areas showing <2 cm increase. A nearby retention basin, in contrast, had water level rises of 100 cm, suggesting that pirating of recharge into stormwater systems occurs. Thus, the concept of an urban karst system as a contaminant conduit is not the only one that applies. In Valley Creek Basin, reduced infiltration due to paving led to smaller storm response and less contaminant input, and the smaller capture area due to diversion of stormwater led to short flow paths and rapid storm response. Although contaminant levels have not increased due to urbanization, the springs may be at risk for future contamination. Short flow paths may reduce flushing, which means that the system will not cleanse itself if contamination occurs.