Evidence for deep sub-surface flow routing in forested upland Wales: implications for contaminant transport and stream flow generation

Abstract

Upland streamflow generation has traditionally been modelled as a simple rainfall-runoff mechanism. However, recent hydrochemical studies conducted in upland Wales have highlighted the potentially important role of bedrock groundwater in streamflow generation processes. To investigate these processes, a detailed and novel field study was established in the riparian zone and lower hillslopes of the Hafren catchment at Plynlimon, mid-Wales. Results from this study showed groundwater near the river behaving in a complex and most likely confined manner within depth-specific horizons. Rapid responses to rainfall in all boreholes at the study site indicated rapid recharge pathways further upslope. The different flow pathways and travel times influenced the chemical character of groundwaters with depth. Groundwaters were shown to discharge into the stream from the fractured bedrock. A lateral rapid flow horizon was also identified as a fast flow pathway immediately below the soils. This highlighted a mechanism whereby rising groundwater may pick up chemical constituents from the lower soils and transfer them quickly to the stream channel. Restrictions in this horizon resulted in groundwater upwelling into the soils at some locations indicating soil water to be sourced from both rising groundwater and rainfall. The role of bedrock groundwater in upland streamflow generation is far more complicated than previously considered, particularly with respect to residence times and flow pathways. Hence, water quality models in upland catchments that do not take account of the bedrock geology and the groundwater interactions therein will be seriously flawed.

Groundwater nitrogen composition and transformation within a moorland catchment, mid-Wales

“…Responses in the intermediate piezometers showed similar variations for all three locations and trends were similar for the shallow and the intermediate piezometers. Haria and Shand (2004) showed that groundwater levels respond in a consistent way at different depths in the adjacent Hafren forested catchment and explained this in terms of a hydrostatic response to rapid recharge from rainfall upslope. It appears that a similar mechanism may be in operation within the moorland catchment.…”

“…Observations in the adjacent Hafren forested catchment show that the response in piezometric heads to precipitation events is very rapid and implies rapid recharge via fracture and bypass flow Shand et al, 2004). Haria and Shand (2004) observed piezometers with artesian flow in the valley bottom and on the lower slopes indicating upward heads close to the river in the Hafren, a forested catchment of the river Severn. Given the fractured nature of the catchment and the steep topography a binary mixing model is proposed for the shallow groundwater system between recent recharge and older, deeper groundwaters.…”

“…Hydrograph separation estimates the baseflow index (proportion of stored water) for the Gwy is 0.34 (Kirby et al, 1991). Groundwater levels in these upland areas, previously considered to be 'impermeable' beneath the soil zone, also show rapid responses to precipitation events, highlighting the importance of groundwater compartments in within-catchment water transport (Haria and Shand, 2006;Haria and Shand, 2004;Shand et al, 2001). Rainfall and stream flow data are shown in Figure 2 …”

“…Groundwater is an important source of base cations in soil pipeflow during storm events, and may lead to an increase in stream base cation concentrations (Chapman et al, 1997). Changes in piezometric heads in the shallow groundwater system and the interconnection between the groundwater, soil water and overland flow in response to rainfall events have shown the dynamic nature of water movement (Haria and Shand, 2006;Haria and Shand, 2004).…”

Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment

“…[5] Hydrometric [e.g., Haria and Shand, 2004] and hydrochemical [e.g., Mulholland, 1993;Burns et al, 1998;Katsuyama et al, 2005;Soulsby et al, 2007] evidence has suggested that groundwater that has infiltrated into bedrock discharges slowly into soil layers and contributes to base flow. Runoff of bedrock groundwater often strongly influences the generation of saturated zones in soil layers.…”