Análisis del discurso

Water infiltrating in the unconfined zone of the Lincolnshire Limestone moves towards the water table under the influence of gravity and may take between minutes and years to reach the water table (Downing and Williams, 1969). After entering the saturated zone, groundwater moves towards areas of discharge. The rate and direction of movement are dictated by the extent of jointing and fissuring and by the lithological composition of the limestone and overlying cover. A diagrammatic cross section of the Lincolnshire Limestone aquifer in the study area is shown in Figure 2.8.

The south Lincolnshire Limestone has an extensively developed fracture pattern, and fissures provide the predominant groundwater flow paths. Fissure paths were enlarged by glacial-age drainage through the limestone to river channels flowing into the Wash (Downing and Williams, 1969), a theory backed by the discovery of extremely old Pleistocene water in the deep confined zone (Downing et a l , 1977). Fissuring and cavities in the unconfined and confined zones were identified by geophysical logging by Marsh (1978). Fissuring is also evident in the numerous quarries in the Lincolnshire Limestone.

In addition to recent water, fissures also contain a component of much older water from the rock matrix (intergranular flow). Intergranular flow from the matrix to fissure waters are driven by chemical concentration gradients (Downing et a l , 1977). The rate of intergranular flow is very low, with laboratory permeabilities <6x10 '^ m/d (Bishop and Lloyd, 1990). This is highlighted by rocks exposed in quarries that show

oxidised (buff) exteriors, where they have been in contact with fissure flow, and

reduced (dark grey) interiors, indicating that the rock has not been completely

oxidised. In this work, such rocks are termed ‘redox blocks’.

Figure 2.8: Diagrammatic cross section of the Lincolnshire Limestone aquifer in the

study area (adapted from Bradbury etal., 1994)

RECHARGE Winter Unconfined | Confined Swallow Hole Recharge \ R. West Summer Unconfined | Confined Abstraction Zone Fenland Winter Summer _{Artesian or} Abstraction Borehole ' ’- / w t r ' -1 0 0 m I I CLAY 8. DRIFT LIMESTONE MUDSTONE ► FLOW DIRECTION WT WATER TABLE 3 0 Km

W Generalised Cross Section

Downing and Williams (1969) estimated the rate of flow in the Lincolnshire

Limestone as between 0.3 m/d and 0.8 m/d. However, they recognised that this was

not a true representation of flow, as fissure, not intergranular, flow dominates. This

was highlighted by a rapid rise in non-carbonate hardness recorded at W ilsthorpe

pum ping station after the onset of recharge, with maximum non-carbonate hardness in

April. W ilsthorpe is 5 km from Limestone outcrop, which led to estimates of localised

flow rates up to 30 m/d. Tracer testing by B ooker (1977) revealed that flow velocities

to the west of the W est Glen ranged from 150 m/d up to 3300 m/d along faulted

zones, with little dilution occurring. However, Bradbury et al. (1994) suggested that some of this rapid flow may have been subterranean flow along the course of the W est

with a large amount o f dilution. It was found that structural barriers often controlled

the m ovement of recharge in the area.

Downing and Williams (1969) found different concentrations of tritium (100

and 68 TU) at different depths (18 m and 22 m), suggesting that vertical mixing of

w ater in the aquifer had not fully occurred, even 13 km from outcrop. However, they

proposed that lateral mixing had occurred over the flow path, since the seasonality of

tritium concentrations seen in the W est Glen River was absent in the boreholes.

B ooker’s (1977) work suggests that this lack of seasonality in the groundw ater is

more likely to reflect mixing of recharge from a number of sources, with variable

transit times.

Figure 2.9: Tritium concentrations (TU) in the south Lincolnshire Limestone aquifer

(Downing et al., 1977) 2 0 9 0 G ra nt ha m 64 'o36 125 3 0 - - 30 Bo u rn e 2 0- - 20 65, 52 - 10 42 P e te rb o ro u g h - 00 'soul km 00 ou tc ro p of th e Lmcolnshire L im est o ne ^ to w n s 250 mg/1 Cl ^ 3 o s i t e s s a m p l e d by I. G .S . ( E d m u n d s 1973)

Downing et al. (1977) found that post-1953 (recent) water extends some 15 km downgradient of the outcrop. Waters containing zero tritium coincided with the 250 mg/1 chloride contour (Figure 2.9). Recent water had penetrated to where all public abstraction was taking place. Also, public abstraction had resulted in the modification of the groundwater pattern by drawing westward the deeper, older water from the east. Aquifer properties for the Lincolnshire Limestone are shown below:

Transmissivity >1500 mVd, locally as high as 5000-10000 mVd (A reduction occurs towards the saline end member) Storage coefficients 5 x 10 ^ to 5 x 10 ^

Well yields typically 5000 m^/d

In summary, flow within the aquifer is predominantly via fissure flow. In the shallow confined aquifer fissure water is predominately recent, but contains a proportion of older water derived from intergranular flow and diffusion. In the deep aquifer the water is of much older age (>25000 years). Groundwater flow pathways are structurally and lithologically controlled. Development of the aquifer for public abstraction has resulted in modification of groundwater flow patterns.