The Role of Bedrock Groundwater in Headwater Catchments: Processes, Patterns, Storage and Transit Time

Abstract

Understanding the role of deep aquifer contributions to headwater rainfall-runoff processes, storages and transit times remains a major challenge in hydrology. Bedrock groundwater contributions to the stream channel can significantly augment streamflow, mediate water quality and control the age of water discharging from catchments. Yet, the hydroclimatic and bedrock characteristics that control these dynamics are not fully understood. Direct observation of bedrock groundwater dynamics, storages and surface water connections remain limited, challenging our ability to fully constrain new catchment scale models that are needed to aid future resource management decisions. I undertook a large field campaign at a well-studied research site in New Zealand. Bedrock groundwater dynamics were monitored for one year and combined with bedrock characterization, tritium-based age dating and hydrochemical analysis to constrain a new conceptual model of the headwater aquifer. Findings were used to develop a new index to identify the controls of bedrock permeability and landscape structure on the time scales of catchment storage-release processes. The three major findings of this research were firstly, that unfractured low-permeability bedrock underlying the research catchment limited to deep flowpaths. Minimal bedrock groundwater flux combined with large bedrock storage resulted in significantly older bedrock groundwater that contributed minimally to catchment discharge. Second, unfractured low-permeability bedrock was a primary control on bedrock groundwater recharge seasonality. Groundwater movement occurred as matrix flow, requiring long durations of high catchment-wetness for considerable recharge to occur, a condition that was only attained during cold-season months when evapotranspiration rates were low and catchment wetness was high. Third, permeability contrasts at the soil-bedrock interface and landscape structure were highly correlated with mean transit time for eight catchments in geologically diverse regions, suggesting that subsurface anisotropy is a major control on setting streamwater age. Overall, through the coupled analysis of the processes, patterns, storages and transit times, this research has advanced our understanding of the role of bedrock groundwater in headwaters. The findings presented here offer new insights into the function of deeper hydrologic layers and have implications for future models of headwater catchment function – models that need to better incorporate the influence of deep flowpaths and storages in groundwater-surface water and rainfall-runoff predictions.