Water Chemical Composition Evolution And Nitrate Source And Migration Pathway In Karst Valley Depression

Due to the excessive development of traditional energy,people pay more and more attention to water energy in recent years.As an important means of effective utilization of water resources,reservoir plays an important role in drinking water,irrigation,flood control and drought relief.However,the use of reservoirs often leads to the interaction of nutrients and hydrological conditions leading to water quality problems,especially due to the special structure of karst areas.Due to the change of hydrological conditions under the influence of dam construction,the exchange of water and pollutants between groundwater and surface rivers in karst areas is fast,and the dynamic of water quality will be more obvious.Karst groundwater provides drinking water for a quarter of the planet’s population.However,in agricultural intensive areas around the world,karst water is generally polluted by nitrate（NO₃^-）,especially in valley depression with good hydrological connectivity.Valley depression aquifers are particularly vulnerable to anthropogenic pollution because their pipes and sinkholes respond quickly to rainfall events and anthropogenic inputs.Identifying the source and migration path of nitrate nitrogen in valley depression is the key to understanding the nitrogen cycle law and effectively preventing NO₃^-pollution.In this paper,a study on the evolution of hydrochemical composition of karst reservoir Muzhu Reservoir（Re）in the upper catchment of Houzhai Catchment was carried out based on the long-term monitoring data set（1981-2002）and the recent years（2013,2015,2016,2017）.2021 and 2022),and collected high-resolution samples at fixed points in the peak cluster depression area（1 surface flow-SS,2 sinking-SH and reservoir-Re）,to explore the source and migration path of nitrate in the peak cluster depression area and evaluate its impact on the water quality of karst reservoirs.Chemical composition concentrations and stable isotopes(δ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-)were analyzed.The R language stable isotope analysis model（SIAR）was used to quantitatively analyze the contribution rate of NO₃^-source.The influence of expansion from underground reservoir to surface reservoir on water quality and nitrate source in karst area was evaluated.The results show that:（1）By analyzing the long-term hydrochemical composition of karst reservoir（Muzhu Reservoir）in valley depression area,the results showed that the hydrochemical type of the water in the study area was changed by reservoir expansion.The hydrochemical type before reservoir expansion was HCO₃^-·SO₄^2--Ca²⁺·Mg²⁺,and that after reservoir expansion was HCO₃^--Ca²⁺.In addition,the expansion of reservoir led to the weakening of water-rock interaction and the increase of nitrate input in water in the short term（1-2 years）,and the decrease of nitrate input in water in the long term（6-7 years）.This indicates that the expansion of the reservoir has a certain impact on the water quality.Combined with the rainfall in the study area,it can be seen that the expansion of the reservoir has increased the nitrate pollution in the water body in the short term（1-2 years）,while in the long term（6-7 years）,the nitrate concentration in the water body has decreased due to the decrease in rainfall,which also indicates that the rainfall in the karst area promotes the nitrogen pollution.（2）The[NO₃^--N]of Muzhu Reservoir at the downstream site was the highest,followed by the sinkholes at the upstream site,and the surface water at the upstream site was the lowest.SIAR source calculation showed that the main source of soil organic nitrogen was soil organic nitrogen,followed by fertilizer and sinkhole in the upstream during non-rainfall period.During rainfall,the main source of soil organic nitrogen was fertilizer,followed by soil organic nitrogen and subsidence pit.Rainfall events accelerate fertilizer infiltration into groundwater.Slight denitrification may have occurred at the sampling site,but assimilation of sow reservoir and drop hole did not occur.In conclusion,agricultural activities are still the main influencing factors of[NO₃^--N]in the study area.（3）By analyzing the nitrate dual isotope composition of karst reservoir（Muzhu Reservoir）in valley depression area,the results showed that the expansion of the reservoir changed the biogeochemical process of the water in the study area.The denitrification effect was greater than the assimilation effect before the expansion of the reservoir,and the denitrification effect was less than the assimilation effect after the expansion.Before and after the expansion of the reservoir,ammonium from fertilizer,soil organic nitrogen,feces and domestic sewage were the main sources of nitrate in water.This indicated that the expansion of reservoir led to the decrease of denitrification in water,and gradually transformed from denitrification to assimilation.Nitrate in water mainly came from ammonium in fertilizer and rainwater,soil organic nitrogen,feces and domestic sewage.The SIAR model of karst reservoir（Muzhu Reservoir）in vally depression area showed that the expansion of reservoir slightly affected the contribution rate of nitrate source.The contribution rate of fertilizer before the expansion of reservoir（42.61%in wet season and 41.39%in dry season）was not significantly different from that after the expansion of reservoir（42.63%in wet season and 41.58%in dry season）.This may indicate that the expansion of reservoir has little effect on the sources of nitrate in water in the short term.In conclusion,the expansion of karst reservoir in the valley depression area has a more obvious influence on water chemistry,while the influence on the contribution of nitrate sources may take a long time to evolve.However,fertilizer,soil organic nitrogen and sinkholes in the upper reaches are still the main sources of nitrate in the water of the study area,and rainfall has a certain promoting effect on nitrate leaching.Therefore,the emphasis of NO₃^-control in the karst valley depression area should be taken into account the fertilization method,fertilization timing and spatial distribution of sinkholes.In order to reduce the nitrogen flux in the valley depression,some effective management measures should be considered,such as extending the water retention time by using wetlands and blocking the nitrogen loss path by using sinkholes.