Assessing acid rain and climate effects on the temporal variation of dissolved organic matter in the unsaturated zone of a karstic system from southern China

Abstract

Assessing acid rain and climate effects on the temporal variation of dissolved organic matter in the unsaturated zone of a karstic Abstract: Acid rain has the potential to significantly impact the quantity and quality of dissolved organic matter (DOM) leached from soil to groundwater. Yet, to date, the effects of acid rain have not been investigated in karstic systems, which are expected to strongly buffer the pH of atmospheric rainfall. This study presents a nine-year DOM fluorescence dataset from a karst unsaturated zone collected from two drip sites (HS4, HS6) in Heshang Cave, southern China between 2005 and 2014. Cross-correlograms show that fluorescence intensity of 2 both dripwaters lagged behind rainfall by ~1 year (~11 months lag for HS4, and ~13 months for HS6), whereas drip rates responded quite quickly to rainfall (0 months lag for HS4, and ~3 months for HS6), based on optimal correlation coefficients. The rapid response of drip rates to rainfall is related to the change of reservoir head pressure in summer, associated with higher rainfall. In winter, low rainfall has a limited effect on head pressure, and drip rates gradually slow to a constant value associated with base flow from the overlying reservoir-this effect being most evident on inter-annual timescales (R 2 = 0.80 for HS4 and R 2 = 0.86 for HS6, n = 9, p < 0.01). We ascribed the ~1 year lag of fluorescence intensity to the effect of the soil moisture deficit and the karst process on delaying water and solute transport. After eliminating the one year lag, the congruent seasonal pacing and amplitude between fluorescence intensity and rainfall observed suggests that the seasonality of fluorescence intensity was mainly controlled by the monsoonal rains which can govern the output of DOM from the soil, as well as the residence time of water in the unsaturated zone. On inter-annual timescales, a robust linear relationship between fluorescence intensity and annual (effective) precipitation amount (R 2 = 0.86 for HS4 and R 2 = 0.77 for HS6, n = 9, p < 0.01) was identified, implying that annual (effective) precipitation is the main determinant of DOM concentration in the aquifer. Conversely, the insensitivity of fluorescence intensity and fluorescence wavelength maxima to variations in the pH of local rainfall suggests that acid rain over the study period (~pH 5.6 to ~4.5) had no discernable effect on the quantity and quality of DOM in karst soil and soil solution, likely being strongly buffered by soil carbonates. Therefore, despite large increases in anthropogenic acid rain in recent Chinese history, hydrologic forcing is the predominant factor driving variations in DOM in karst aquifers.

East Asian Precipitation δ18O Relationship With Various Monsoon Indices

“…According to the meteorological data of Yichang station (1950( -2008, about 90 km away from the study site, the monthly air temperature varied from 1.5°C to 30.6°C, with the lowest temperature in January and the highest temperature in July and August, and an average annual air temperature of around 16.9°C. The mean annual precipitation amount recorded at Yichang was 1,162 mm, and approximately 70% of the annual precipitation occurred between May and September (Liao et al, 2018). In addition to Heshang Cave, the HS site is located near other caves where long-term records of speleothem δ 18 O have been published, including Sanbao Cave (Cheng et al, 2016;Wang, Cheng, et al, 2008) and Qingtian Cave (Liu et al, 2008) in Shennongjia, Lianhua Cave (Cosford et al, 2008), Yaoba Don Cave (Jason et al, 2008), and Jintanwan Cave (Cosford et al, 2010) in Hunan, and Furong Cave (Li et al, 2011) in Chongqing (Figure 1b).…”

Variation Characteristics of the Main Hydrochemical Indexes in Typical Subterranean Rivers in the South China Karst Region Based on Curve Fitting

“…This observation can be attributed to the dissolution of carbonate rock [19], which is mainly due to the greenhouse effect and water acidification. In recent years, the majority of the precipitation in the South China Karst region has mostly been acidic [20]. Furthermore, the greenhouse effect and water acidification can also cause the dissolution of dolomite, releasing Mg ions into solution.…”

Drying increases organic colloidal mobilization in the karst vadose zone: Evidence from a 15‐year cave‐monitoring study

“…Details of the monitoring and sampling setups are provided by Liao et al (2018). In this section, we provide a brief review and focus on the details that relate specifically to the study.…”

“…In this section, we provide a brief review and focus on the details that relate specifically to the study. (Liao et al, 2018) and separately by a well-trained local resident (Hu, Henderson, Huang, Chen, & Johnson, 2008).…”

“…The difference in the emission fluorescence spectra of raw and filtered (0.22 μm) HS4 samples at the above wavelengths can thus represent the concentration of COM quantitatively (Figure 2; Baker et al, 2007;Coble et al, 2014). HS4 dripwaters were filtered through 0.22 μm with polycarbonate membrane syringe filters (Millipore; Liao et al, 2018;Seidel et al, 2014). The fluorescence spectra of the raw and filtered samples were measured through two-stage experiments at the China University of Geosciences (Wuhan).…”