Böhlke J K, O'Connell Michael E, Prestegaard Karen L
USGS, 431 National Center, Reston, VA 20192, USA.
J Environ Qual. 2007 Apr 5;36(3):664-80. doi: 10.2134/jeq2006.0084. Print 2007 May-Jun.
Ground water processes affecting seasonal variations of surface water nitrate concentrations were investigated in an incised first-order stream in an agricultural watershed with a riparian forest in the coastal plain of Maryland. Aquifer characteristics including sediment stratigraphy, geochemistry, and hydraulic properties were examined in combination with chemical and isotopic analyses of ground water, macropore discharge, and stream water. The ground water flow system exhibits vertical stratification of hydraulic properties and redox conditions, with sub-horizontal boundaries that extend beneath the field and adjacent riparian forest. Below the minimum water table position, ground water age gradients indicate low recharge rates (2-5 cm yr(-1)) and long residence times (years to decades), whereas the transient ground water wedge between the maximum and minimum water table positions has a relatively short residence time (months to years), partly because of an upward increase in hydraulic conductivity. Oxygen reduction and denitrification in recharging ground waters are coupled with pyrite oxidation near the minimum water table elevation in a mottled weathering zone in Tertiary marine glauconitic sediments. The incised stream had high nitrate concentrations during high flow conditions when much of the ground water was transmitted rapidly across the riparian zone in a shallow oxic aquifer wedge with abundant outflow macropores, and low nitrate concentrations during low flow conditions when the oxic wedge was smaller and stream discharge was dominated by upwelling from the deeper denitrified parts of the aquifer. Results from this and similar studies illustrate the importance of near-stream geomorphology and subsurface geology as controls of riparian zone function and delivery of nitrate to streams in agricultural watersheds.
在马里兰州沿海平原一个具有河岸森林的农业流域内，对一条下切的一级溪流中影响地表水硝酸盐浓度季节变化的地下水过程进行了研究。结合地下水、大孔隙排水和溪水的化学及同位素分析，研究了包括沉积物地层学、地球化学和水力性质在内的含水层特征。地下水流动系统呈现出水力性质和氧化还原条件的垂直分层，其水平边界延伸至田地和相邻河岸森林下方。在最低地下水位位置以下，地下水年龄梯度表明补给率较低（2 - 5厘米/年）且停留时间较长（数年至数十年），而最高和最低地下水位之间的瞬态地下水楔的停留时间相对较短（数月至数年），部分原因是水力传导率向上增加。在第三纪海相绿泥石沉积物的斑驳风化带中，补给地下水中的氧还原和反硝化作用与最低地下水位附近的黄铁矿氧化作用相关联。在高流量条件下，下切溪流的硝酸盐浓度较高，此时大部分地下水在一个具有丰富流出大孔隙的浅有氧含水层楔中迅速穿过河岸带；而在低流量条件下，硝酸盐浓度较低，此时有氧楔较小，溪水流量主要由含水层较深的反硝化部分向上涌流主导。这项研究以及类似研究的结果表明，近溪流地貌和地下地质作为农业流域河岸带功能及硝酸盐向溪流输送的控制因素具有重要意义。
