Karst hydrogeology of the Chalk and implications for groundwater protection

The Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m.d.−1. Tests to abstraction boreholes also demonstrate very rapid velocities of 1000s m.d.−1. Natural gradient tests from observation boreholes have rapid velocities of 100s m.d.−1. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Palaeogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones.

Hydrochemistry and Evolution of Water Quality in a Context of Aridity and Increasing Agriculture in Three River Sub-Basins of Santiago Island (Cape Verde)

In regions under development and facing recurrent droughts, increasing the area of irrigated agriculture may create additional disruption in water resources management. The present study was focused on three river sub-basins with the highest agricultural intensity (S. Miguel, Ribeira Seca and S. Domingos) in Santiago Island (Cape Verde). Sets of wells were selected to evaluate the influence of salinization and agriculture practices on the hydrochemistry. This assessment was performed by using data from the bibliography (2003) and a recent campaign (2016). The water chemistry indicates lower mineralization in the S. Miguel sub-basin. Nitrates and nitrites, typically associated with diffuse pollution, are present in all sub-basins, but with varying patterns. Additionally, sodium chloride waters occur in all the three sub-basins, especially those closest to the coastline. In turn, a bicarbonate-magnesium facies was identified in S. Domingos, at the furthest point from the coast, indicating a geological control. The comparison between the two periods suggests a decrease in water quality. The rising extension of the irrigation area associated with aridity should intensify the already observed soil salinization. Thus, the present review highlights the strategic importance of water monitoring at the basin level as a management tool for resources preservation in insular arid and developing regions.

‘Hidden Hot Springs’ as a Source of Groundwater Fluoride and Severe Dental Fluorosis in Malawi

Hidden hot springs likely impact rural water supplies in Malawi’s Rift Valley with excess dissolved fluoride leading to localised endemic severe dental fluorosis. Predicting their occurrence is a challenge; Malawi’s groundwater data archive is sporadic and incomplete which prevents the application of standard modelling techniques. A creative alternative method to predict hidden hot spring locations was developed using a synthesis of proxy indicators (geological, geochemical, dental) and is shown to be at least 75% effective. An exciting collaboration between geoscientists and dentists allowed corroboration of severe dental fluorosis with hydrogeological vulnerability. Thirteen hidden hot springs were identified based on synthesised proxy indicators. A vulnerability prediction map for the region was developed and is the first of its kind in Malawi. It allows improved groundwater fluoride prediction in Malawi’s rift basin which hosts the majority of hot springs. Moreover, it allows dentists to recognise geological control over community oral health. Collaborative efforts have proven mutually beneficial, allowing both disciplines to conduct targeted research to improve community wellbeing and health and inform policy development in their respective areas. This work contributes globally in developing nations where incomplete groundwater data and vulnerability to groundwater contamination from hydrothermal fluoride exist in tandem.

The Influence of Reef Topography on Storm-Driven Sand Flux

Natural formations of rock and coral can support geologically controlled beaches, where the beach dynamics are significantly influenced by these structures. However, little is known about how alongshore variations in geological controls influence beach morphodynamics. Therefore, in this study we focus on the storm response of a beach (Yanchep in south Western Australia) that has strong alongshore variation in the level of geological control because of the heterogeneous calcarenite limestone reef. We used a modified version of XBeach to simulate the beach morphodynamics during a significant winter storm event. We find that the longshore variation in topography of the reef resulted in: (1) strong spatial difference in current distribution, including areas with strong currents jets; and (2) significant alongshore differences in sand flux, with larger fluxes in areas strongly geologically controlled by reefs. In particular, this resulted in enhanced beach erosion at the boundary of the reef where strong currents jet-exited the nearshore.

Rivers Try Harder. Reversed “Differential Erosion” as Geological Control of Flood in the Large Fluvial Systems in Poland

We study cross-sections on the Detailed Geological Map of Poland (SMGP) to find a geologic and geomorphic pattern under river valleys in Poland. The pattern was found in 20 reaches of the largest Polish rivers (Odra, Warta, Vistula, Narew, and Bug) located in the European Lowland, in the landscape of old (Pleistocene, Saalian) glacial high plains extending between the Last Glacial Maximum (LGM) moraines on the North and the Upland on the South. The Upland was slightly folded and up-faulted during Alpine orogeny together with the thrust of Carpathian nappes and the uplift of Tatra Mts. and Sudetes. The found pattern is an alluvial river with broad Holocene floodplain and the channel developed atop the protrusion of bedrock (Jurassic, Cretaceous limestones, marlstones, sandstones) or non-alluvial, cohesive, overconsolidated sediments resistant to erosion (glacial tills, lacustrine or “ice-dammed lake” clays) of Cenozoic (Paleogene, Neogene, Quaternary—Elsterian). We regard the sub-alluvial protrusion as the limit of river incision and scour. It cannot be determined why the river flows atop these protrusions, in opposition to “differential erosion”, a geomorphology principle. We assume it is evidence of geological flood control. We propose an environmental and geomorphological framework for the hydrotechnical design of instream river training.

Assessment of Groundwater Geochemistry of Vel River basin, Western Maharashtra, India

To study the chemistry of major ion in groundwater from Vel (Velu) River basin, sixty (60) samples of dug wells and bore wells were collected and analyzed using standard techniques given by APHA. It shows order of dominance for cations, Na+ > Ca2+ > Mg2+ > K+ and in anionic concentration as HCO3- > Cl- > SO42- in groundwater. The pH of groundwater is slightly alkaline (range: pH 7.0 - 8.1), while average values of Electrical Conductivity (EC) is about 2641 µS/cm indicating high mineralization of groundwater. In general, the cationic concentration (Na+, K+, Ca2+ and Mg2+) of the groundwater increase in the downstream side (from Northwest to South east), suggesting geological control on the composition of groundwater while highest concentration is in lower part of the basin are generally associated with the high salinity. In the major anions, bicarbonate (HCO3-) is higher due to rock-water interaction. Average value of chloride is about of 235 mg/L due to discharge zones along with anthropogenic activities. The geochemical data plotted on Piper Trilinear Diagram is showing dominant hydro-chemical facies: Ca2++Mg2+, Na++ K+, Cl-+ SO42- -HCO3- found in 83.3 % samples indicating the alkaline earth exceeding the alkalis and the strong acids exceeds the weak acids. The pH, Total Hardness (TH) and Magnesium (Mg2+) of the samples show more proportion of samples falling above desirable limit. Otherwise the quality of groundwater is good for drinking. The irrigation indices like SAR, KR and SSP were considered to evaluate groundwater suitability for irrigation. Comparing with SAR parameter all samples are excellent to good for irrigation. In SSP, 33.3 % samples are within permissible, while 66.6% samples are doubtful for irrigation purpose. In KR almost all samples (excluding 04 samples in lower side of basin) are suitable for irrigation. So, variations in climate, geology with anthropogenic activities are modifying the groundwater geochemistry of Vel River Basin.