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Hydrochemical behaviour and dissolved carbon dynamics have high sensitivity to hydrologicvariations during high-flow periods in monsoonal-influenced regions, especially inthe critical zone of agricultural karstic catchment where existing anthropogenicinterference and carbonate weathering with rapid kinetics. Here we selected Chenqicatchment, a 1.5 km2 sub-catchment of Houzhai basin in Puding, SW China, as studyarea to analyze the response of hydrochemistry and dissolved carbon dynamicsto discharge changes, through hourly sampling campaigns in the outlet of springwater during the high-flow period. Most dissolved ions exhibit distinct responsesto discharge variations, their concentrations are negatively correlated with springwater discharge, which can be well fitted using power-law function, indicating theirchemostatic behaviours as discharge increases, but biogeochemical processes influencingdifferent ions are miscellaneous. Ca2+, Mg2+ and HCO3− are mainly controlled bydissolved soil CO2 and carbonate weathering, while NO3− and Cl− both haveanthropogenic inputs and may exist some link about their source areas or transport processesactivated during high-flow periods. Other ions are generally affected by the dilution ofrainwater. Hydrological conditions and biogeochemical processes regulate dissolvedcarbon dynamics, resulting in the variation of δ13CDIC. There are two sources forDIC in spring water of Chenqi catchment, namely biological source (DICbio) andgeological source (DICcarb). The contribution ratio of DICbio is around thrice that ofDICcarb, and they both show chemostatic response to varying discharge althoughthe degrees for DICbio is stronger. The response of dissolved carbon dynamics todischarge variations display different degrees at different stages of the wet season. Itwas find that most dissolved solutes (except DOC) normally present clockwisehysteresis loops plotting from the top-left to bottom-right due to the dilution onthe rising limb of the storm, but the loop area and hysteresis index are differentwithin or between diverse storms. The pattern is related to rain intensity/amount andantecedent conditions, which can affect flow pathways and likely source areas, leadingto different transport time and loads. These hysteresis behaviours are generallycontrolled by antecedent conditions of storms, range/peak of discharge and soluteconcentrations, and storm duration. Overall, this study highlighted the sensitivecharacteristics of hydrochemistry and dissolved carbon dynamics to hydrologicalvariations in small agricultural karstic catchment based on high resolution information. [ABSTRACT FROM AUTHOR]

Extensive anthropogenic activities have resulted in increased delivery of reactive nitrogen to rivers and streams, which can cause aquatic pollution and increase human health risks. Thus, it is important to identify the sources and fate of nitrate for water quality management. Stable isotopes of nitrate have been widely used to evaluate biogeochemical behavior of nitrate in aquatic ecosystem. However, isotopic fractionation and low-resolution monitoring limit the accurate estimation of nitrate sources. Here, we collect river water and conduct a high-time resolution measurement of isotopic analysis (δ15N-NO3− and δ18O-NO3−) in the Xijiang River, Southwest China. The Xijiang watershed covers approximately 77.8% of the drainage basin area of the Pearl River watershed. It has abundant water resources, with an average discharge of 2.30 × 1011 m3 yr−1. The studied watershed belongs to a subtropical humid monsoon climate with annual air temperature changing from 14°C to 22°C. The mean annual precipitation varies from 1,200 mm to 2,200 mm with mainly concentrated from April to September (wet season). The contribution ratios of nitrate sources are calculated using a Bayesian isotope mixing model (SIAR) by incorporating isotopic fractionation. The δ15N-NO3− and δ18O-NO3− vary seasonally with higher δ15N-NO3− (+8.4 ± 1.6‰) and lower δ18O-NO3− (+2.0 ± 1.4‰) in the dry season compared with those of wet season (+7.6 ± 1.6‰ for δ15N-NO3− and +2.7 ± 1.4‰ for δ18O-NO3−). The isotope values of NO3− vary greatly under different discharge conditions in the wet season, highlighting the importance of high-time resolution measurement especially in the wet season. The outputs of Bayesian model indicate that soil organic nitrogen and chemical fertilizer contribute the most (72−73%) in the wet season; whereas approximately 58% of nitrate is derived from manure and sewage wastes and chemical fertilizer in the dry season. This study combines hydrochemistry, stable isotopes with multi-statistical method, and make important progress in accurate estimation of nitrate dynamics in river. [ABSTRACT FROM AUTHOR]

Without careful land management, agricultural land in karst systems can pollute water courses, with polluted waters travel quickly to and through the sub-surface in karst systems. However, detailed understanding of how rapidly nitrate moves within the karst critical zone (from soils to aquifers) is limited by low resolution sampling for this highly-transmissive system. To understand nitrate behavior and its controlling factors, we deployed sensor technology at five sites to generate autonomously high-resolution time series of discharge and [NO3−N] in a farmed karst catchment in SW China. The [NO3−N] time series exhibited rapid response to rainfall-induced increases in discharge and a large magnitude in [NO3−N], ranging from 0.72 to 16.3 mg/L. The highest mean [NO3−N] and normalized annual fluvial export were observed in one headwater catchment with well-developed conduit structure. Elsewhere this was less-pronounced due to buffering by the karstic aquifer network as the contributing catchment area increased with distance downstream. Clear seasonal variation in NO3−N export occurred in response to source availability, most notable in catchments with valley agriculture: up to 97% of nitrate was exported from the headwater catchment in the wet season, with much of that loss occurring in only a couple of months, compared to 61% of nitrate exported at the larger catchment scale. The sensor time series showed that the aquifers were chronically polluted with nitrate. [ABSTRACT FROM AUTHOR]

A significant population within the Lower Volta River Basin of Ghana, an important international tourism destination, relies solely on untreated groundwater and surface water (SW) for various purposes. However, negative practices associated with increasing anthropogenic activities pose threats to especially groundwater in the basin. This study therefore explored an effective integrated approach to assess the origins of groundwater NO3- contamination and related human health risks using hydrochemistry, CCME model, multivariate statistics, isotopes (δ2H, δ18O, and 15N) and USEPA human health risk model. Hydrochemical results showed a slightly acidic to alkaline SW and groundwater environments. Maximum EC of 15850 µS/cm with mean, 2929 µS/cm and elevated values > 1000 µS/cm were observed in 45% of the groundwater across the basin. NO3- levels in the groundwater ranged from 0.12 – 733 mg/l with average 59.59 of mg/l while a maximum of 5.33 mg/l was observed for SW. A total of 75% and 35 % of the sampled groundwater exceeded background NO3- level of 2.1 mg/l and WHO guideline value (45 mg/l), suggesting anthropogenic influence on its quality. The CCME model revealed that, 85% of the SW was of excellent quality. Only 13% of the groundwater ranked good. NO3-/Cl- ratio indicated that 43% and 21% of the groundwater and SW respectively are contaminated by effluents and agrochemicals. NO3- positively correlated with K+ > Ca > Mg2+ > Cl- > Na+ > EC and PCA clearly defined trends of groundwater NO3- pollution with minimal natural attenuation. Manure, septic effluent and mineralized fertilizers were sources of NO3- contamination in the basin. Overall, 70%, 50% and 48% medium to high NO3- risk levels were determined for infants, children and adults with 79% high risk of SW NO2- contamination to infants. Appropriate management options to control groundwater NO3- pollution in the basin were proposed. [ABSTRACT FROM AUTHOR]