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Non-point source (NPS) pollution has caused serious threats to water quality on a global scale. However, the investigation using a single measure with multi-scenarios for the long-term simulation in karst agricultural watershed is still lacking. In this study, the Annualized Agricultural Non-Point Source Pollution (AnnAGNPS) model was developed to verify the applicability in a karst agricultural watershed. Then, the model was used to determine the best management measures (BMPs) and the reduction rate characteristics under long-term effects (about 10 years) in the watershed. The AnnAGNPS model perform well in simulating in NPS pollution with R2 (0.95 for runoff, 0.93 for TN, and 0.93 for TP, respectively) and NSE (0.95 for runoff, 0.53 for TN, and 0.57 for TP, respectively). The output of total nitrogen (TN) and total phosphorus (TP) primarily occurred in the rainy season (up to 80%). The loss of mass of TN and TP were mainly observed in orchards and woodlands in the upstream of each sub-basin. The results from AnnAGNPS model demonstrated that different BMPs had significant impacts on the reduction of NPS pollution. Furthermore, a same BMPs measure showed it was closely related to land use in the watershed. In the Qifeng River watershed, stubble tillage (ST) showed to be useful with relatively good reduction rates (16.64% for sediment, 17.85% for TN, and 17.80% for TP, respectively). The simulation results indicated that AnnAGNPS was a valuable tool after validation for the planning and management of the watershed in karst areas.

Debate continues over the reducing mechanisms for the formation of unconformity‐related uranium (URU) deposits. This paper evaluates, for the first time, the potential of iron‐rich chlorite as a reductant for uranium mineralization using reactive fluid flow modeling method. Our results confirm that Fe2+, released from the breakdown of iron‐rich chlorite, can reduce aqueous hexavalent uranium to precipitate economically significant URU deposits similar in size and grade to those formed with CH4 as the reducing agent. The resulting uranium mineralization tends to occur in the basement and below the downwelling parts of overlying basinal fluid circulation cells, where oxidizing basinal fluid percolates across the unconformity and reacts with upward flowing reducing basement brine. Therefore, the basinal fluid circulation pattern controlled by the permeability of the sandstone aquifer is critical in determining the formation and distribution of URU deposits. When the sandstone layer is more permeable, the simulated uranium deposits become larger in size, and vice versa. If the sandstone permeability is <5 × 10−14 m2, no obvious uranium deposits can be formed. In contrast, the permeability of fault zones does not have a significant effect on uranium mineralization, although it does affect fluid flow behaviors within the fault zone itself. We also demonstrate that fault zones do not appear to be a prerequisite for the formation of URU deposits when Fe+2 serves as a reductant, which highlights important exploration implications. Uranium exploration should, in addition to continuing to target graphitic fault zones, also consider areas where faults and/or graphite units do not exist. Plain Language Summary: This study uses reactive fluid flow modeling to assess the potential of Fe‐rich chlorite as a reductant for the formation of unconformity‐related uranium (URU) deposits. The results demonstrate, for the first time to our knowledge, that Fe2+ can reduce hexavalent U to precipitate uraninite comparable in size and grade to those formed by CH4. In the case of Fe2+ as the reducing agent, fault zones do not appear to be necessary for uranium mineralization of economic significance. Therefore, uranium exploration should also consider areas where fault zones and/or graphite units do not exist, but where there are extensive sources of bivalent Fe present. Key Points: First reactive flow modeling study on the role of ferrous iron as a reducing agent for the formation of unconformity‐related uranium depositsFe2+ derived from iron‐rich chlorite can form uranium deposits similar in size and grade to those formed by CH4 as a reducing agentUranium exploration should also consider areas where fault zones and/or graphite units do not exist [ABSTRACT FROM AUTHOR]