Abstract:

Shallow aquifers experience water table fluctuations that push and pull
water and air through organic-rich soils, which affects the redox state
of soils and influences groundwater quality. Here, I present column
experiments and numerical models to understand the fate of nitrogen (N)
and carbon (C) in two settings subject to flooding and water table
fluctuations: a relatively pristine, forested mountain stream site (Font
del Regàs, Spain) and a developed coastal site (Matarò, Spain). For each
site, we filled a meter-long column with a sequence of natural organic
topsoil and aquifer sediments. A fluctuating head was imposed at the
bottom of each column, and local groundwater was introduced with its
natural nitrate concentration (0.48 and 16.5 mg/L nitrate-N,
respectively, at the forest stream and coastal sites). In the coastal
sediments, the organic-rich topsoil remained anaerobic, while mineral
sediments beneath alternated between aerobic, when the water table
dropped and sucked air across preferential flow paths, and anaerobic
conditions suitable for denitrification when the water table was high.
Conversely, in the forest setting, the coarse nature of upper riparian
soils ensured more aerobic conditions at the organic soil layer. Organic
layers acted as a source rather than a sink of nitrate when soils were
dry but switched to reducing conditions that favored denitrification
when soils were saturated. Flooding the soils from above at both sites
delivered nitrate to the saturated zone. These controlled experiments
illustrate the wide range of redox conditions that prevail in organic
soil layers within floodplains and their effects on nitrate sources and
sinks as the water table rises and falls.