Hydrology and groundwater nutrient concentrations in a ditch-drained agroecosystem

Abstract:

Groundwater nitrogen (N) and phosphorus (P) transport from ditch-drained, cultivated fields has not been adequately investigated in the Chesapeake Bay watershed. We monitored hydrology and groundwater N and P concentrations in 26 shallow (~ 3 m [10 ft]) wells for 27 months on a heavily ditched, poultry-grain farm on Maryland's Lower Eastern Shore. Water tables fluctuated above and below shallow ditches, but were always higher than deep ditches. Thus, groundwater flow to shallow ditches was intermittent, but flow to deep ditches was continuous. Water tables rose rapidly with rain, but drained back from 15 to 60 cm (6 to 24 in) the first day after rain. The rate of water table fall decreased rapidly thereafter. Water tables frequently perched on top of subsoil clay horizons. Although perching persisted only 24 to 48 hours, nutrient transport could be accelerated if rapid, lateral movement of water to ditches occurs. Frequent and widespread concentrations of groundwater NO₃-N greater than 10 mg L⁻¹ show subsurface N loss from the farm is probable. High concentrations of dissolved P existed in groundwater, but P movement in groundwater was restricted. Rain infiltrating through topsoils mobilized soil P into groundwater and moved considerably high concentrations of P as deep as 1.5 m (4 ft), where elevated P concentrations persisted for days or weeks. Groundwater P concentrations were greatest where high water table hydrology combined with the greatest soil P concentrations. Delivery of groundwater P to shallow ditches was apparently controlled by near-ditch soil P conditions, while P delivery to deep ditches was controlled by how deep groundwater flowed. Therefore, limiting soil P accumulation in near-ditch zones may help reduce P delivery to shallow ditches, and increasing the length of groundwater flow paths through low-P subsoils may help reduce P delivery to deep ditches.