The Origins of Deep Groundwaters in Cumbria, Northwest England: Hydrochemical Constraints

Abstract

In order to determine groundwater flow paths it is essential to characterise the origins of the waters and their dissolved solute load. This is often easier for shallow groundwaters in relatively permeable sedimentary cover sequences than it is for deep groundwaters in less permeable crystalline basement rocks. Deep waters will usually have longer residence times within the geosphere than shallower waters, and often will have undergone extensive water/rock interactions. At localities where cover rocks overstep basement rocks, groundwater origins and residence times are influenced by the possibility of hydraulic connectivity between the cover and the basement. Such a locality occurs at the eastern margin of the East Irish Sea Basin (EISB), at Sellafield, northwest England (Figure 1). Here, relatively permeable Carboniferous to Triassic sedimentary cover rocks are underlain by less permeable Ordovician volcanic basement rocks of the Borrowdale Volcanic Group (BVG). Data for deep (up to 1.7 kin) groundwaters at this location have been produced recently during the investigation of the site for a potential nuclear waste repository, which is being undertaken by U.K. Nirex Ltd. Additionally, analyses of porewaters from bo t tom sediments in a freshwater lake, Derwentwater, to the northeast of Sellafield (Figure 1), have been reported. The purpose of this paper is to assess these new hydrochemical data in order to characterise the relationship betweenl~l~groundwaters in the cover and basement rocks of the area. Two main types of groundwater occur in the Sellafield area (U.K. Nirex Ltd, 1993): 1. shallow, eq!ute (< c. 500 mgl -I Total Dissolved Solids (,~DS)) Ca-HCO3 and Na-Ca-HCO3 dominated waters; and 2. deeper, more saline, Na-C1 dominated waters. In the EISB, waters of the first type occur only in the cover, and in the very top of the basement towards the basin margin in the east. Waters of the second type occur at depth both in the cover and in the basement, and their salinities increase from c. 30000 mg/l Total Dissolved Solids (TDS) in the east, to c. 180000 mg/l towards the basin centre in the west. The first type of water is interpreted to have been recharged relatively recently and to have acquired its salinity mainly through water-rock interactions involving carbo-