Soil humic substances (SHS) are heterogeneous, complex mixtures, whose concentration, chemical composition, and structure affect the transport and distribution of heavy metals. This study investigated the distribution behavior of two heavy metals [cadmium (Cd) and nickel (Ni)] in high molecular weight SHS (HMHS, 1 kDa–0.45 μm) and low molecular weight SHS (LMHS, <1 kDa) extracted from agricultural soils. The HMHS mass fractions were 45.1 ± 19.3%, 17.1 ± 6.7%, and 57.7 ± 18.5% for dissolved organic carbon (DOC), Cd, and Ni, respectively. The metal binding affinity, unit organic carbon binding with heavy metal ratios ([Me]/[DOC]), were between 0.41 ± 0.09 μmol/g-C and 7.29 ± 2.27 μmol/g-C. Cd preferred binding with LMHS (p < 0.001), while Ni preferred binding with HMHS (p < 0.001). The optical indicators SUVA254, SR, and FI were 3.16 ± 1.62 L/mg-C/m, 0.54 ± 0.18 and 1.57 ± 0.15, respectively for HMHS and 2.65 ± 1.25 L/mg-C/m, 0.40 ± 0.17, and 1.68 ± 0.12, respectively for LMHS. The HMHS contained more aromatic and lower FI values than LMHS. Multilinear regression showed a significant positive correlation between the measured predicted [Me]/[DOC] ratios (r = 0.52–0.72, p < 0.001). The results show that the optical indices can distinguish the chemical composition and structure of different size SHS and predict the binding ability of Me-SHS.

: The composition and structure of dissolved organic matter (DOM) are sensitive indicators that guide the water infiltration process in soil. The DOM chemical composition in seepage affects river water quality and changes soil organic matter (SOM). In this lysimeter test study, fluorescence spectra and optical indices were used to examine the interaction between the percolation water (P-W) and leachate water (L-W) DOMs affected by the soil solution (S-S). The L-W DOM had a higher aromaticity (SUVA254), average molecular weight (S275-295) and terrestrial source (fluorescence index (FI)), but fewer autochthonous sources (biological index (BIX)) than the P-W DOM. Organic carbon standardization (OCS) and protein- (PLF), fulvic- (FLF) and humic-like fluorescence (HLF) intensity showed that L-W DOM increased 44%, 55% and 81%, respectively, compared to the P-W DOM. The linear regression slopes between OCS FLF and PLF were 0.62, 1.74 and 1.79 for P-W, L-W and S-S, respectively. The slopes between OCS HLF and PLF were 0.15, 0.58 and 0.64 for P-W, L-W and S-S, respectively. The P-W DOM was in contact with the soil litter layer, where S-S labile lignin phenolic compounds released and dissolved into the L-W DOM. This increased its aromaticity, and extent of humification.

The groundwater resources in the Pingtung Plain are crucial water sources in southern Taiwan. However, they have been significantly impacted by climate change, resulting in changes in groundwater quality and quantity in the region. To effectively manage groundwater extraction, this study utilized runs theory to analyze the safe groundwater levels at six groundwater level observation stations located in the proximal fan, mid fan, and distal fan areas of the Pingtung Plain. The methodology involved dividing the range between the maximum and minimum groundwater levels at each station into 20 equal intervals. The groundwater levels were then sorted in ascending order, and the cumulative frequency percentiles of groundwater levels in each interval were calculated to determine the truncation levels for runs theory. Subsequently, the groundwater over-extraction duration and severity were computed. By comparing the results with the groundwater management levels set by the Water Resources Agency of the Ministry of Economic Affairs, it was found that the safe groundwater levels in the proximal fan and distal fan areas were the average of observation data plus 0.5 times the standard deviation. The over-withdrawn duration for these areas was approximately 8 to 10 months and 8 months, respectively. In the mid fan area, the safe groundwater level was based on the average of observation data, and the over-withdrawn duration ranged from 6 to 9 months.