Characteristics and Influencing Factors of Nitrogen Loss in Guancun Karst Subterranean River Catchment
|Keywords||subterranean river nitrate nitrogen loss hydrology water chemistry|
Nitrate pollution is one of prevalent and serious dangerous pollutants for groundwater resources.Superfluous nitrate in groundwater will harm the people health and the crop.The WHO regulates nitrogen should not exceed 11mg/l for safe drinking water,and in China it should not greater than 20 mg/l（NO3--N）.Prevent and father nitrate contamination of groundwater is one of attention issues for scholar.Concentration of nitrate in shallow groundwater should less than 4.4 mg/l in natural condition according to statistic,therefore it is consider groundwater was affected by anthropogenic activities when nitrate greater than 4.4 mg/l.The source of nitrate contamination is mainly from extensive use of fertilizer and pesticide,industry waste,septic systems and poultry,hog or cattle manure.There are 2836 subterranean rivers having been found in south China,with a total discharge of 1482 m3/s.Groundwater resources from these subterranean rivers are important production and potable water for the local people.Yet along with intensive agriculture and industry,urban expansion, subterranean rivers suffer from various degree contaminations,which are threaten by turn to sewer. Therefore,a case study for typical subterranean rivers in karst areas in south China is very important by using advanced auto-monitored water chemistry technology.The annually sendout of nitrogen from the catchment and the seasonally change of them and factors affected nitrogen loss were discussed.All of these should have a great significance for preventing and fathering subterranean rivers contamination.Auto-monitored apparatus for water chemistry and flux observation weir were set up in the outlet of subterranean river,from which long-term of auto monitor having been carried out for several years.Nitrate was analyzed for one per day and water chemistry for other ions monthly,from which seasonal variation of nitrate and major ions were discussed.The quantity of nitrogen loss and rate were figured out.The ratio of nitrogen input and output from the subterranean river were figured out too.Influencing factors were discussed according to natural factors and anthropogenic activities.Dynamic variation of water chemistry in rainy and dry season showed that subterranean rivers system were different from epikarst spring.The flux rise and fall did not quickly like epikarst due to big drainage area than them,indicating that the complexity of recharge process.Guancun subterranean river system respond to all of the rainfall in a year were auto-recorded by apparatus. The variation characteristic of Guancun subterranean river system in short time scale was understood.Factors such as rainfall,intensity,duration and flux increase speed influenced and reflected the sensitivity degree of system.There were three recharge effects,namely,sinkhole recharge,fissure recharge and invalidation recharge could be distinguished and judged by these indexes synthetically.Generant conditions of three recharge types were summed up.The results were very important for understanding the function of subterranean river system.Water type of Guancun catchment was Ca-HCO3.Ca2+and HCO3- were the major component, secondly were Mg2+,Na+,K+,Cl-,SO42-and NO3-.Besides that the others were few.The range of pH value was 6.95-7.71.The variation range of HCO3- was 3.5-4.9mol/l,with a character of high in winter and low in summer.CO2 pressure of groundwater was ranged in 4900-14500ppm.Observation for eight ions in yearly and heavy rain process revealed that the concentration of sulfate and nitrate had a peak in rainy season when the concentrations of sodium,calcium, magnesium and bicarbonate were low.The concentrations of potassium and chloride changed randomly throughout the year.The concentration of nitrate in flood process was rot completely controlled by discharge.Only the concentrations of nitrate and sulfate had obviously increased during the past two decades.It was believed that dilution,eluviation,karst erosion and anthropogenic activity can explain the ion variations.According to three years observation from June 2004 to June 2007,nitrate was in peak value in early rainy season.This phenomenon appeared repeatly in three years.Peak value commonly appeared from Feb.to May.Low value appeared from October to December.Peak value of nitrate appeared before rain peak,the reason was nitrogen accumulated in soil in winter were eluviated to underground in early rainy season,then it become stabilization after dilution by heavy rain. Moreover,late Mar.to early Apr.was a period of centralized cropland fertilization.Peak values of nitrate in three years were not equation incompletely due to rainfall influence.Two functions controlled the variation of nitrate in heavy rain process.Firstly it was eluviation, then dilution.The behavior of it was ion concentration increase quickly.Then it decrease gradually after soil saturation.Finally come back to normally value.Variation of ions concentration in small rainfall mostly controlled by dilution,showing that ion concentration decreased a little when rain began,then resumed to a normally value or did not affect by them.The range of nitrate variation was 2.56-15.40mg/l,with an average value of 5.60mg/l.Nitrate concentrations were obviously increase compared with those of 1978 and 1980.Three facts were considered to be responsible for the change:（1）The change in fertilizer.In the former period only organic manure made of excrement was used in the fields,and now the fertilizer was popular;（2） Increase in the amount of livestock.The number of goats in the catchment had reached 3000,with a result of great manure spreading in pasture;（3）Reduction in vegetation and soil erosion.The vegetation was the only source of energy before use of firedamp.Soil erosion accelerated on the slopes without the protection of vegetation.The quantity of nitrogen input and output from May,2006 to May,2007 in Guancun subterranean river were figured out.The quantity of nitrogen input was 66.61t,and the output was 21.24t.The reason of nitrogen unbalance was volatilization,absorption by soil or existing in other form.The primary reason of nitrogen loss was rainfall and anthropogenic activities.Nitrogen output from Apr.to Jun.made up for 57 percent in a year.And water discharge of that made up for 59 percent.Nitrogen loss due to natural factor accounted for 76.11 percent in a year,while nitrogen loss due to anthropogenic activities accounting for 23.89 percent.Nitrogen loss was compared with other drainage area in other background of the world,showing that nitrogen loss was decided by infield area,crop species,fertilizer quantity and it’s manage manner mostly.While study on soil thickness in karst area and how development of surface karst form effected nitrogen loss shoud do more.