Bunter Sandstone Aquifer Eastern England

The Bunter sandstone aquifer has been studied over an area of 2000 km2 in eastern England (Fig. 11.18). A large body of data has been published by Andrews and Lee (1979) and Bath et al. (1979). The regional geology, shown in Fig. 11.19, reveals a sequence of formations from the Permian in the west to the Jurassic in the east, the strata dipping eastward. A more detailed geology of the study area, with sampled well locations, is given in Fig. 11.20, and a cross-section is given in Fig. 11.21. The researchers suggested that water is recharged at the Bunter sandstone outcrops and moves eastward, down-gradient, into a confined section.

Fig. 11.18 Study area of the Bunter sandstone in eastern England: 1—area shown in Fig. 11.19; 2—area shown in Fig. 11.20. Topographic elevations (masl) are indicated. (Following Mazor and Kroitoru, 1987.)
Fig. 11.19 Generalized geological map of a section of east England. (Following Mazor and Kroitoru, 1987.)

The reported data have been plotted in composition diagrams in order to determine whether it is really one continuous aquifer. The Cl-d18O plot (Fig. 11.22) reveals three data clusters, indicating that three specific water groups—A, B, and C—occur. The Cl-14C plot (Fig. 11.23) also reveals three

Fig. 11.20 Geology and location of sampled wells in the study area of Bunter sandstone aquifer. (Following Bath et al., 1979.)

Fig. 11.21 Suggested geological cross-section of the Bunter sandstone aquifer. The researchers suggested that water recharged in the western outcrops flows eastward down-gradient into the confined section. (Following Bath et al., 1979.)

groups, but the data of group A are spread along a line, interpreted as a mixing line of a recent saline component (possibly contaminated with anthropogenic chlorine) and a several-thousand-year-old, chlorine-poor, end member. The three geochemical groups have a simple geographical distribution pattern of north-south strips, as seen in Fig. 11.24. Group A coincides with the belt of phreatic Bunter sandstone outcrops, whereas groups B and C are in the confined, east-ward-dipping section. The 14C, Cl, and He data maps of Fig. 11.24 reveal two discontinuity lines—I and II. These lines are parallel with the strike of the rock strata and formation contacts (Figs. 11.20 and 11.21). Thus there is little or no active through-flow from the phreatic zone of group A to the confined zone of group B.

Fig. 11.22 Cl—d18O plot of the Bunter sandstone wells. Three distinct geochemical groups emerge: A, B, and C. (Data from Bath et al., 1979.)
Fig. 11.23 Cl- C plot of the Bunter sandstone wells. Again, three groups emerge. The group A points plot along a line, interpreted as a mixing line of recent (postbomb, also tritium containing) saline (polluted?) water with several-thousand-year-old fresh water. (Data from Bath et al., 1979.)

Furthermore, a discontinuity also occurs in the confined zone, and the group B section does not communicate freely with the group C section.

The Bunter sandstone dD and d18O data provide further support for the lack of communication between the phreatic and confined aquifer sections. The data plot in Fig. 11.25 along a line, but the data reveal an order of B?A?C. In other words, one is dealing with three distinct water groups, revealing different environmental conditions that prevailed at the time of recharge.

The three water groups of the Bunter sandstone have distinct ages, as revealed by two independent age indicators, 14C and 4He (Fig. 11.26). The positive 4He-14C ages correlation gives the age grouping a high degree of confidence. From Fig. 11.26 it seems that water group C has for a long time been separated from group B, but the discontinuity between A and B is also maintained in Fig. 11.26.

Bearing in mind the concept of stagnant groundwater prevailing beneath the level of the terminal base of drainage (section 2.13), let us have a second look at the geological cross-section of the Bunter sandstone aquifer (Fig. 11.21). The rock strata dip eastward in the direction of the sea, attaining depths of hundreds of meters below sea level. Thus, stagnation is

Fig. 11.24 C, He, Cl, and temperature data marked on the well location map (Fig. 11.20). The three distinct geochemical groups—A, B, and C—already defined in Figs. 11.22 and 11.23, have a clear geographical pattern: group A coincides with the phreatic wells in the Bunter sandstone outcrop area, whereas groups B and C are in the confined section. The data mark two discontinuity lines, I and II, discussed in the text. (Data from Bath et al., 1979.)

Fig. 11.24 C, He, Cl, and temperature data marked on the well location map (Fig. 11.20). The three distinct geochemical groups—A, B, and C—already defined in Figs. 11.22 and 11.23, have a clear geographical pattern: group A coincides with the phreatic wells in the Bunter sandstone outcrop area, whereas groups B and C are in the confined section. The data mark two discontinuity lines, I and II, discussed in the text. (Data from Bath et al., 1979.)

to be expected and is supported by the discussed chemical and isotopic data. A glance in Fig. 11.18 reveals that the topographic relief of the suggested recharge area is very shallow, only a few tens of meters above sea level. Thus, no extra-high hydraulic pressures can be developed that could push groundwater through theoretical U-tubes in the Bunter sandstone. It seems plausible that the wells of zone A (Figs. 11.22-11.26) tap a through-flow system (phreatic aquifer), whereas the wells of zones B and C tap stagnant systems (fully confined aquifers).

Fig. 11.25 dD—d18O plot of the Bunter sandstone wells. The obtained pattern is discussed in the text. (Data from Bath et al., 1979.)

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