Effect of Transient Hydrologic Events on Biogeochemical Processes

在对一系列水文地质环境中的地下水进行的实地研究中，微生物介导的地球化学反应的速率已被证明远远快于水文传输速率。这些相对速率导致含水层内各种对氧化还原反应敏感的化学物质在短距离内的浓度发生较大变化。然而，在一些系统中，瞬时水文事件(例如，干旱土壤上的大量降水或溪流中的洪水)可以迅速改变微生物生存的地球化学环境。在这种情况下，与物理事件相关的时间常数接近于微生物中介过程的时间常数。拟议的研究涉及以下问题：水文过程和生物地球化学过程如何在河流的河岸-浅水区相互作用，在那里各种过程的时间尺度都在同一量级--几小时到几周？这项拟议的研究将考察河岸蓄水事件对低地势沿海平原溪流河岸-浅水地带发生的还原微生物过程的影响。特别是，入河地下水中硝态氮的浓度约为15 mg/L，而溪水的平均浓度仅为1.6 mg/L，表明河岸-浅水区具有较高的反硝化活性。我们设想在降雨过程中，厌氧过程会被破坏，在这种情况下，快速的渗透会提高地下水位，并将孔隙水中的化学物质(即硝酸盐)“冲入”溪流，并向该地区灌入含氧水。我们预计，在暴风雨期间，如果没有冲洗，也会出现类似的破坏，即水位的快速上升将含氧水推入河岸，从而短暂地延长低渗漏区。我们还将研究冲刷或银行储存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比率。由于这些瞬变事件有可能影响硝酸盐等化学物质从地下水向周围地表水的释放，我们将调查河岸土壤中的瞬变条件如何影响生物活性化学品的总体预算。拟议的工作包括详细的野外观测、野外操纵实验、实验室批处理和“中观”实验以及数学建模。我们预计，我们将学习微生物介导的地球化学反应和时间尺度相同的瞬变水文过程是如何联系在一起的。我们将对这些过程进行定量描述，并调查不同水文事件“重置”生物地球化学过程的频率，我们将研究局部过程的瞬时效应如何影响区域营养盐通量的天气数据。

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.