Including the Effects of Groundwater Flow in Thermochronological Studies: A Sensitivity Analysis of the Thermal Effects of Recharge Rates

Elco Luijendijk¹, Marlies ter Voorde¹, Ronald van Balen¹, Hanneke Verweij², Jan Diederik van Wees², and Erik Simmelink^2
1Institute of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV, Amsterdam
²TNO Built Environment and Geosciences, PO Box 80015, 3508 TA, Utrecht

As numerous studies have demonstrated, groundwater flow can have a significant impact on subsurface temperatures, and therefore also on exhumation histories derived from thermochronological studies. Our research aims to quantify this influence by including fluid flow in numerical models for the thermal behaviour of the crust and sediments. These models will be applied to the Roer Valley Graben in the southern Netherlands. One of the parameters to consider in this research are variations in recharge and discharge rates in the area. Commonly, numerical simulations of groundwater flow on basin scales represent the upper model boundary by a fixed hydraulic head at the position of the water table. This approach does not impose any constraints on recharge and discharge rates at the upper model boundary and could therefore lead to unrealistic estimates of the thermal effects of groundwater flow.
To evaluate the potential effect of recharge on subsurface temperatures a parameter sensitivity study was conducted using a 2D numerical model of groundwater and heat flow. As a first step, the sensitivity analysis was conducted on a synthetic 20 km wide and 12 km deep basin, with a variation in water table elevation of 100 m. Preliminary results indicate that recharge can have a significant effect on subsurface temperatures. Using best estimates of recharge and permeability, the simulated advective cooling reaches values of up to 10 °C, and is mainly confined to the upper 2 km of the model domain. A fixed hydraulic head boundary condition can lead to values of groundwater recharge which are unrealistic given best estimates of the bandwith of recharge during geological history. Permeabilties of the upper part of the model of ≤10-14 m² result in unrealistically low values of recharge (less than 10 mm/yr), while permeabilities of ≥10-11 m² can result in recharge values in excess of 1000 mm/yr. These recharge rates can lead to underestimation or overestimation of advective cooling that exceed a factor of 4. Both recharge and the thermal effects of recharge are highly sensitive to the variation of permeability with depth. The sensitivity of recharge to anisotropy and water table elevation is much lower, although both parameters can still have a large impact on subsurface temperatures. The preliminary conclusion is that we will have to incorporate a more realistic representation of recharge in the model for our research-objectives.
In addition to an improved description of recharge, we will incorporate the influence of faults and of groundwater salinity gradients on fluid flow. The models will be used to quantify to what extent fluid flow can explain subsurface temperatures as well as paleo-geothermal gradients inferred from apatite fission tracks from several borehole samples. Constraints will be based on a large geological dataset including approximately 100 boreholes and over 1000 km of seismic lines, as well as data and a large number of previous studies on stratigraphy, fluid flow during geological history and subsurface temperatures. The effects of groundwater flow on the thermal structure of the crust will be compared to the effect of other thermal processes, such as conduction, advection, heat production, topographic changes, fault movements, erosion and sedimentation.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands