| The karst water fixates atmospheric CO2as dissolved inorganic carbon through carbonate rock dissolution with water-rock interaction. But when karst groundwater exposes to the surface, calcium carbonate will be deposited as the form of travertine、tufa or stalagmites, and CO2will be released to the atmosphere, so karst carbon sequestration effect has also been questioned. For the purpose of a more comprehensive and objective understanding on karst carbon sequestration, the temporal and spatial variation of the HCO3-concentration was monitored after exposed to the surface, also carbon sequestration flux of the karst spring catchment and degassing flux of its stream was estimated.In this study, water temperature、conductivity pH、dissolved oxygen and Ca2+、HCO3-concentration were measured in situ, anions、cations concentration and the carbon stable isotope values were analyzed in lab. With high-density、high temporal resolution long-term monitoring at Baishuwan Spring and its downstream, variation of karst stream hydrochemical characteristics along the flow path was revealed, its controlling factors were explored, especially the factors affecting degassing of the karst stream. Finally carbon sequestration flux of the spring drainage and degassing flux of the karst stream was estimated. The result showed that:(1) There was no big change in Baishuwan spring water chemistry, except for the differences in dry season and rainy season, pH values、conductivity、Ca2+、HCO3-concentration were higher in dry season, while lower in rainy season, which might be caused by the dilution effect of rainfall. The discharge of the spring was among0.04～5.70L/s, the average flow was about0.79L/s, water temperature was14.4～16.8℃, the average value was16.0℃, pH values were7.22～7.82, conductivity was somewhere in between536～594μs/cm; ion composition was mainly HCO3-and Ca2+, whose proportion in anion and cation was81.7%、92.2%respectively, and SO42-was accounted for16.13%of the anion, hydrochemical type of the spring water is HCO3-Ca type. After the spring was exposed to the surface, Ca2+、HCO3-concentration、 conductivity and pCO2of the downstream water presented obvious law of diminishing along the flow path, due to the degassing effect; while pH values、dissolved oxygen、calcite saturation index increased gradually, but no significant variation was found in other conventional anion and cation.(2) Variation of the karst stream water chemistry was influenced by multiple factors, including degassing effect of the stream、calcium carbonate deposition、water temperature、discharge, etc, among these, degassing was the dominant factor.In addition, the stream water chemistry presented different variations in different months, due to differences in water temperature and discharge. Calcium carbonate deposition was also an important factor affecting the stream water chemistry, conversely, the stream water chemistry affected calcium carbonate deposition profoundly. From Jan. to Mar.2012,pCO2values at the spring outlet were lower than other months, concentrations of Ca2+was higher, and discharge of the stream was smaller, coupled with the hydrological disturbance in the stream drainage, these were beneficial for travertine deposition, so significant deposition of calcium arbonate was found in the stream drainage during this period.(3) Factors affecting degassing effect in the karst stream included seasonal discharge of the spring、 CO2partial pressure at the spring outlet、the stream water temperature、biological role、hydraulic gradient of the stream drainage. Seasonal discharge of the spring was not only decisive factor of the total degassing flux, but also affected the rate of degassing, expressed as "drought period>normal period>rainy period". Moreover, free CO2concentration differences between the upstream and downstream indicated that CO2partial pressure at the spring outlet was the main factor influencing CO2out gassing rate, when pCO2value was higher at the spring outlet, free CO2concentration differences were greater, indicating that the degassing rate was greater. Data showed that there was a good positive correlation between the stream water temperature and degassing flux, in the daytime, since the water temperature was higher, the solubility of CO2in water was reduced as the temperature increased, and the degassing flux was larger.Diurnal monitoring results showed that the biological role also had a certain influence on degassing effect:1. Decreasing rate of HCO3-concentration along the flow path was "midday> morning> midnight", but stream water temperature was lower in the morning than midnight, it was caused by photosynthesis of algae in stream drainage and plants around, free CO2in the stream was uptaken and utilizated, as a result, degassing rate in the morning was greater conversely.2. Vegetation was poor among s1～s8stream segments, the biological role was relatively weak, that’s why no obvious diurnal variation of HCO3-concentration at s8sampling sites was showed, however significant diurnal variation of HCO3-concentration was found at s11sampling sites.3. The concentration of CO2in the air close to the stream surface performed as "midnight> morning> midday", this was because biological photosynthesis dominated at midday, CO2was uptaken and utilizated, and the concentration of CO2in the air close to the stream was lowest.4. Rate of δ13CDIC values increasing along the flow path also presented as "midday> morning> midnight", and circadian fluctuation of δ13CDIC values at s1(the spring outlet) was small, while δ13CDIC values at s8(161m away from the spring outlet) and s11(260m away from the spring outlet) fluctuated obviously,δ13CDIC values in the daytime were heavier than the nighttime.In addition, the study found that the s8-s9drainage segments had shorter length, comparing to s6-s8, but had larger hydraulic gradient, as a result, the HCO3-concentration reduction was greater. This indicated that stream turbulence and stream drainage slope accelerated degassing effect by intensifying hydrological disturbance.(4) Estimation results about carbon sequestration flux of the spring catchment and degassing flux of its stream demonstrated that, the recycled atmospheric CO2by Baishuwan Spring averaged about15.97kgCO2per day during monitoring period, and the degassing flux of its stream averaged about3.58kgCO2per day. Consequently, even though degassing effect occurred after the karst groundwater was exposed to the surface, degassing flux of the stream only accounted for about22.4%of the carbon sequestration flux within the range of the studied stream, thus there was still a considerable amount of net carbon sequestration. |
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