Effect of Transient Hydrologic Events on Biogeochemical Processes

[20886 . 86]在一系列水文地质环境中对地下水进行的实地研究表明，微生物介导的地球化学反应的速率远远快于水文运输速率。这些相对速率导致含水层内短距离内各种氧化还原敏感化学物质的浓度发生巨大变化。然而，在某些系统中，短暂的水文事件（例如，干燥土壤上的强降水或溪流中的洪水）可以迅速改变微生物存在的地球化学环境。在这种情况下，与物理事件相关的时间常数接近微生物介导过程的时间常数。拟议的研究解决了这样一个问题：水文过程和生物地球化学过程是如何在河流的河岸-潜流带相互作用的，其中各种过程的时间尺度是相同的-小时到周？拟议的研究将检查河岸储存事件对低起伏海岸平原河流的河岸-潜流带发生的还原性微生物过程的影响。尤其值得注意的是，流入河流的地下水中硝酸盐- n的浓度约为15 mg/l，而河流水的平均浓度仅为1.6 mg/l，表明在河岸-暗沉带具有较高的反硝化活性。我们设想，在降雨期间，厌氧过程被破坏，快速渗透提高了地下水位，将孔隙水中的化学物质（即硝酸盐）“冲洗”到流中，并向该地区注入含氧水。我们预计，在风暴期间也会出现类似的破坏，但不会出现冲刷现象，在这种情况下，河流水位的迅速上升将含氧水推入河岸，从而短暂地扩大了暗流带。我们还将研究冲洗或银行存储事件对反硝化的影响，并将记录微生物群落功能能力恢复到干扰前水平的速率。由于这些瞬态事件有可能影响硝酸盐等化学物质从地下水释放到周围的地表水中，我们将研究河岸土壤的瞬态条件如何影响生物活性化学物质的总体预算。建议的工作包括详细的实地观察、实地操作实验、实验室批量和“中观”实验以及数学建模。我们预计，我们将了解微生物介导的地球化学反应和瞬态水文过程在同一顺序的时间尺度是如何联系在一起的。我们将对这些过程进行定量描述，并研究生物地球化学过程被不同水文事件“重置”的频率。我们将研究局部过程的短暂效应如何影响区域营养通量。

0208386MillsIn field studies of groundwater in a range of hydrogeological settings, the rates of microbially mediated geochemical reactions have been shown to be far faster than hydrological transport rates. These relative rates lead to large changes in concentration of a variety of redox-sensitive chemical species over short distances within aquifers. In some systems, however, transient hydrological events (e.g., heavy precipitation on dry soil or floods in streams) can rapidly alter the geochemical environment in which the microbes exist. In that circumstance, the time constants associated with the physical events approach those of the microbially mediated processes. The proposed research addresses the question: How do hydrological processes and biogeochemical processes interact in the riparian-hyporheic zone of streams where time scales of the various processes are on the same order - hours to weeks? The proposed research will examine the effect of bank-storage events on reductive microbiological processes occurring in the riparian-hyporheic zone of a low-relief coastal plain stream. In particular, the concentration of nitrate-N in the groundwater feeding the stream is about 15 mg/liter, but the stream-water concentrations average only 1.6 mg/liter, suggesting high denitrification activity in the riparian- hyporheic zone. We envision the disruption of anaerobic processes during rain events in which rapid infiltration raises the water table and "flushes" chemicals (viz., nitrate) in the pore water out into the stream and perfuses the area with oxygenated water. We expect a similar disruption without flushing during storms in which a rapid rise in stream level pushes oxygenated water into the stream banks, thereby transiently extending the hyporheic zone. We will also examine the effect of flushing or bank-storage events on the denitrification and will document the rate of return of the functional abilities of the microbial communities to the pre-disturbance levels. Because these transient events have the potential to affect release of chemicals like nitrate from the groundwater into the surrounding surface waters, we will investigate how transient conditions in riparian soils affect the overall budget of biologically active chemicals. The proposed work includes detailed field observations, field manipulation experiments, laboratory batch and "mesocosm" experiments and mathematical modeling. We anticipate that we will learn how microbially mediated geochemical reactions and transient hydrological processes with time scales on the same order are linked. We will develop quantitative descriptions of these processes and investigate how frequently biogeochemical processes are "reset" by different hydrological events the synoptic data that we will examine how the transient effects of local processes influence regional nutrient fluxes.