Groundwater monitoring of an open-pit limestone quarry: Water-rock interaction and mixing estimation within the rock layers by geochemical and statistical analyses

Water-rock interaction and groundwater mixing are important phenomena in understanding hydrogeological systems and the stability of rock slopes especially those consisting largely of moderately watersoluble minerals like calcite. In this study, the hydrogeological and geochemical evolutions of groundwater in a limestone quarry composed of three strata: limestone layer (covering), interbedded layer under the covering layer, and slaty greenstone layer (basement) were investigated. Water-rock interaction in the open-pit limestone quarry was evaluated using PHREEQC, while hierarchical cluster analysis (HCA) and principal component analysis (PCA) were used to classify and identify water sources responsible for possible groundwater mixing within rock layers. In addition, Geochemist's Workbench was applied to estimate the mixing fractions to clarify sensitive zones that may affect rock slope stability. The results showed that the changes in Ca2+ and HCO3 concentrations of several groundwater samples along the interbedded layer could be attributed to mixing groundwater from the limestone layer and that from slaty greenstone layer. Based on the HCA and PCA results, groundwaters were classified into several types depending on their origin: (1) groundwater from the limestone layer (L-o), (2) mixed groundwater flowing along the interbedded layer (e.g., groundwater samples L-7, L-11, S-3 and S-4), and (3) groundwater originating from the slaty greenstone layer (S-o). The mixing fractions of 41% L-o: 59% S-o, 64% L-o: 36% S-o, 43% L-o : 57% S-o and 25% L-o: 75% S o on the normal days corresponded to groundwaters L-7, L-11, S-3 and S-4, respectively, while the mixing fractions of groundwaters L-7 and L-11 (61% L-o: 39% S-o and 93% L-o: 7% S-o, respectively) on rainy days became the majority of groundwater originating from the limestone layer. These indicate that groundwater along the interbedded layer significantly affected the stability of rock slopes by enlarging multi-breaking zones in the layer through calcite dissolution and inducing high water pressure, tension cracks and potential sliding plane along this layer particularly during intense rainfall episodes

Accessing the Impact of Floating Houses on Water Quality in Tonle Sap Lake, Cambodia

The floating houses in Tonle Sap Lake might be one of the main factors for degradation of water quality since the people in floating houses discharge sewage and waste from their households into the lake. Therefore, the government of Cambodia has decided to move the floating houses in Chhnok Tru to the upland regions, and more than 90% of the floating houses in Chhnok Tru have already been moved in accordance with the government’s plan. However, the scientific information on water quality before and after moving the floating houses in Tonle Sap Lake is limited. Thus, this paper aimed to evaluate differences in basic water quality such as temperature, pH, dissolved oxygen (DO), oxidation–reduction potential (ORP), conductivity (Cond), and nitrate (NO3−) before and after the floating houses were moved and to reveal the relationships between the floating houses and basic water quality. The water quality parameters were measured at 18 sampling sites in Chhnok Tru using an EXO sensor and NO3− was analyzed by ion chromatography (IC). Statistical analyses such as t-tests, correlation analysis, principal component analysis (PCA), and structural equation modeling (SEM) were used. The results show that the water quality was better after moving the floating houses; however, some parts of the study area were still polluted. In addition, the percentage of floating house distribution was significantly correlated with the temperature and ORP in the study area during dry and wet seasons. The obtained results are useful for making management decisions to sustainably manage the water quality in the area

A Multi-Method Approach to Flood Mapping: Reconstructing Inundation Changes in the Cambodian Upper Mekong Delta

International audienceAs in many tropical deltas globally, annual floods shape the livelihoods of the largely rural population in the Cambodian Mekong delta. Agricultural cycles are keyed to the flood arrival, peak, and recession, and fish populations depend on inundated floodplains for their regeneration. However, as factors like climate change and hydropower infrastructure development are altering the Mekong's hydrology, the inundation dynamics of its deltaic floodplains are shifting as well. Several studies have assessed the general changes of river discharge and flood extent on a basin-or delta-wide scale. Yet the sustainable development of this region is relying on dynamics at more local and specific scales, which have not been addressed so far. This paper presents a methodology to track the evolution of hydrological regimes and associated inundations in tropical deltas such as the upper Mekong delta in Cambodia, where it is applied over the past 30 years. Data scarcity and heterogeneity of the environment in this region necessitated the use of combined approaches. We established a link between water levels measured in situ and flood maps derived from optical and radar satellite images (Sentinel-1 and -2).The robustness of the link was assessed using Sentinel, Landsat imagery and the TanDEM-X (12 m) elevation model. This water level-flood link (WAFL) was then used to reconstruct a daily time series of inundation extents reaching back to the beginning of hydrological measurements in 1991 (30 years). On this basis, changes in the incidence, duration, and spatial distribution of floods were analysed. The results indicated that WAFL can be used to reconstruct inundation maps with an overall robustness of 87% in comparison to historical inundation maps derived from remote sensing imagery. Comparisons of WAFLderived flood extents with Landsat images further underscored the significant role of local infrastructure, sedimentation dynamics, and land cover to explain changes in inundation dynamics. WAFL-based analyses revealed that inundation durations have decreased by an average of 19 days when comparing the periods before and after 2008, which was identified as a break point in the hydrological time series. Furthermore, a drastic decrease in inundation the annual frequency with which individual pixels are flooded can be detected during the first half of the traditional flood season, with an average of-21% in early August, negatively impacting water-based livelihoods, from agriculture to fisheries