The Ef fect of B o r e h o l e S e a l i n g on t h e S o u t h e r n L i n c o l n s h i r e L i m e s t o n e C a t c h m e n t D. Johnson, MSc, CEng, MICE, K. R . Rushton, DSc, CEng, MICE (Member) and L. M. Tomlinson, BSc*

Abstract

The Southern 1,incolnshire Limestone is an important groundwater resource; however, for many years water has been lost from the aquifer system through uncontrolled artesian flow from wild-bores in the Fenland area. During 1991-92, there was an extensive programme of sealing or controlling the wild-bores, and compensation flows for ecological and farming purposes were provided at some sites.

This paper describes a field study which was designed to identify the important features of part of the catchment; this was followed by the development of an existing groundwater model to provide an improved representation of the uncontrolled and controlled wild- bores. The model is used to assess the consequences of controlling or sealing the wild-bores.

Key words: Boreholes; groundwater; limestone aquifer; Southern Lincolnshire.

In t roduct ion

This paper is concerned with the surficc-water catch- ment and associated aquifers to the west of the South Forty Foot drain in Lincolnshire. The catchment is underlain by the major aquifer of the Southern Lincoln- shire Limestone which dips from unconfined conditions on the scarp slope in the west to confined conditions under the Fens in the east. The aquifer is an important resource for public water supply and maintaining surface-water flows which support irrigation. In the Fens, many boreholes were drilled through into the Lincolnshire Limestone to supply water to the farms. Most of these boreholes were abandoned and, due to the artesian pressure in the underlying aquifer, water escaped from these bores. These overflowing 'wild-bores' provide part of the surface-water flows - especially in times of limited rainfall - and have also been significant in supporting the region's ecology.

In order to examine surface-water conditions in the catchment of the South Forty Foot drain, it is necessary to identify the various sources of surface water in addition to the wild-bores. A study of the surface-water hydrology shows significant flows from the minor aqui- fers which overlie the Southern 1,incolnshire 1,imestone. This paper includes a preliminary study which considers how the various sources of the water can be identified.

Since the 1970s, the overflowing wild-bores have been seen as a potential loss of water from the aquifer

and, in 1991-92, a major project to control and seal the wild-bores was carried out('). In the controlled boreholes, a reduced flow is released to provide a continuing source of high-quality water to maintain sensitive ecosystems which have developed over the years. However, effective- ness of the programme of wild-bore control has not yet been studied.

In this work, the flow mechanisms of wild bores were investigated, and an improved method of represen- ting wild-bores was introduced. This, and other develop- men&, have led to an improved simulation of the whole Southern Lincolnshire I .imestone catchment.

The groundwater model has allowed an assessment as to whether controlling the wild-bores has led to a significant reduction in the wild-bore flows and, if so, how this 'saved' water leaves the aquifer system. Another important question is whether the controlling of the wild-bores has reduced the likelihood of drawing in poor- quality water from the east.

S u r v e y o f W e s t e r n P o r t i o n o f South F o r t y Foot D r a i n C a t c h m e n t

There are two distinctive parts of the Southern Ihco ln - shire Iimestone catchment (Fig. 1): the unconfined area to the west where the water-table lies within the lime- stone, and the confined portion to the east where artesian pressures often occur. The western part of the catchment

b, +A"aakby +Dunmby ri South Forty

Ist t Foot Drain Catchment A ;len

W Fig. 1. Important features of Southern Lincolnshire Limestone

'Research student, protessor and research fellow, respectively, School of Engineering. University of Birmingham, UK

0 J.CIWEM,1999. 13, F e b r u a r y 37

0. Johnson, K. R . Rushton and L. M. Tomlinson on

is drained by the East and West Glen rivers, whilst the eastern portion lies within the catchment of the South Forty Foot drain.

To the west of the South Forty Foot drain there are high-level carriers taking runoff and drainage water from the Overlying Reds and low-level carriers primarily taking water from the Fens. Fig. 2 is a schematic diagram of the main carriers and sources of water, and a full list of the wild-bores can be found in Barton and Perkins(').

Geology

catchment is illustrated in Fig. 3. A representative geological cross-section of the

360

330

300

2 F 3 P

270

240

210

The Upper Lias Clay, which is part of the Lower Estuarine Series, forms an effectively impermeable base to the 1,incolnshire Limestone which is 20-40 m thick; the eastwards extent of the 1,imestone is uncertain. The lithology is of a varying calcareous rock consisting of shell beds, oolites, pisolites, cementstones, coral knobs and sandy limestones.

Formations overlying the 1,incolnshire 1,imestonc have a significant effect on both surface and sub-surface flows; they will be called the Overlying Reds and consist of an alternating sequence of aquifers and low per- meability beds. The lowest member, the Upper Estuarine Series, H hich lies directly above the Lincolnshire Lime-

I I I I I I 1 I I I 1

80 100 120 140 160 180 Easting

Fig. 2. Schematic diagram of system of high- and low-level carriers in South Forty Foot drain catchment

38 0 J.CIWEM,1999, 13, Februa ry

The Effect o f Borehole Sea l ing o n the Southern L inco lnsh i re L imestone Catchment

B.Cl Boulder Clay n ri nvfnrd rhv BL Lm Blisworth Limestone

BLC1 IMs~Orth clay 60.0 -,

---.r

K.B Kellaway Be& USE upper Estuarine C.Br Cornbrash L.Lm Lincolnshire Limestone

L.E Lower Estuarine A1 Alluvium

40.0

rn AOD SOUTH FORTY FOOT DRAIN CAfCH#EMI

20.0 - \ \\>

O . O - r - - - - - - -Y

-20.0

-40.0

-60.0 --

Fig. 3. Representative cross-section showing important features of geology of South Forty Foot drain catchment

stone, consists of up to 14 m of grey-green clays, shales and marls with local patches of thin limestones and white sands. Above the Upper Estuarine series lies the Blisworth Limestone (Great Oolite Limestone) which consists of various calcareous lithologies varying from massive limestones to thin marl-clay horizons. Another minor aquifer, the Cornbrash, is separated from the Blisworth Iimestone by the Blisworth Clay. The upper- most strata of the Overlying Beds are the Kellaway Beds and Oxford Clay which extend over large areas in the east of the study area. There are extensive Boulder Clay deposits over most of the study area and also Fen deposits to the east.

Natural Springs Part of the catchment in the South Forty Foot drain

was studied in detail to learn more about the natural springs (Fig. 4) which feed into high-level carriers. One of the strongest is to the NNE of Folkingham ('W 074343), which occurs on the Blisworth Limestone a t approximately 40m AOD. More evidence of springs includes an inn with a well in its cellars, which now requires a sump pump to prevent flooding. Hand-pumps are also evident in many of the villages.

Other natural springs to the east of Folkingham, such as Horbling Farm and West Laughton (TF 076 313), occur on the Cornbrash outcrops; some of the springs are perennial apart from during dry summers. The springs at Bromwell (TF 115 285) and Little Dowsby (TF 109304) are further examples of Cornbrash-fed springs which have strong winter (but weak summer) flows.

Lincolnshire has a large number of ancient and holy

wells. In the study area the oldest is at Sempringham Priory (TF 107 328) where the spring is on, or near to, an outcrop of Cornbrash. The church springs at Billing- borough (TP 118 342), which feed into a pond adjacent to the church, have only failed four times in recent years.

The well at Horbling, which was constructed in 171 1 and discharges to a series of stone troughs, appears to be founded on (or near to) an outcrop of Cornbrash. The water has similar properties to Billingborough Church springs in that it does not stain the stonework, although de-gassing is evident.

Artesian Boreholes and Wild-Bores In the nineteenth century, as farms spread out onto

the Fens, many artesian boreholes were sunk through the Oxford Clay and Overlying Beds into the Ihcolnshire Thes tone aquifer for domestic and agricultural use. These bores were often lined with poor-quality material which deteriorated, leaving behind a leaking borehole which could not be controlled. As small farms were amalgamated, boreholes were abandoned with the un- controlled discharge entering the Fen-drainage system where some of the water was used for irrigation and the remainder was discharged into the South Forty Foot drain. Many of the sources have since been abandoned; and there is a likelihood that water might flow up from the Lincolnshire Limestone and enter permeable over- lying strata (such as the Blisworth Limestone and the Cornbrash) or enter the surface-drainage system.

Known boreholes were controlled or sealed in the late 1980s and early 19YOs, with a compensation flow allowed from certain wild-bores to maintain valuable aquatic ecosystems which had developed over the years.

0 J,CIWEM,1999, 13, February 3 9

D Johnson. K . R . Rushton and L. M . Tomlinson on

370

360

350

340 cn C

r

0

.- 4 - 3 3 0 L

5 2 0

310

300

290

50 60 70 80 90 100 110 1zO 130 140 150 160 170 180 190

E a s t i n g

Fig. 4. Location of natural springs and wild-bores in Billingborough Fen to Folkingham catchments

Fenland Drainage The drainage of the Fens began with the Romans

who constructed Car Dyke to intercept surface water from the west before it meandered across the Fens in a series of small rivulets. During the seventeenth century, this system of drainage was replaced by the Dutch system of field drains with a network of porous underdrains discharging into dykes. The water in the dykes flows by gravity into a series of pumping stations which raise the water into the South Forty Foot drain so that it can flow by gravity to the sea. During the summer, the dykes are also used to retain drainage to maintain a high water-table and to provide a reservoir for irrigation.

Levels in the dykes are controlled by sluices, non- return valves and pumping, and the elevations of sluices at the exit of each drainage ditch are set to allow drainage by gravity, whenever possible. At all other times, water is lifted by low-head pumps into the South Forty Foot drain. During winter, the water levels in the dykes are maintained below the level of the field drains to allow efficient drainage. In the summer, the water level is maintained at 750 mm below the level of the lowest land (i.e. about 1 m below average ground level). The drainage returns of the Black Sluice Internal Drainage Board (BSIDB) show the amount of water pumped out of each Fen. If the water level in the dyke is higher than the South Forty Foot drain, the dyke drains under gravity. Although the pumping returns can be used to investigate the operation of the Fens, they cannot be used to estimate the wild-bore flows because there are many other com- ponents in the dyke water balance, including runoff due to rainfall, abstraction for irrigation, and the storing of water for agricultural purposes(').

C o n t r o l of W i l d - B o r e s

Construction of Original Boreholes The construction of many of the original wild-bores was with hand- or steam-driven boring machines which were used to sink a 50-mm shaft to depths of up to 60 m. The shaft was lined with a mild-steel pipe which was drawn down as the bore was sunk. A proportion of the outflow from the wild boreholes can therefore occur from the Cornbrash and Rlisworth Limestones, provided that corrosion of the lining pipes has occurred adjacent to these two minor aquifers.

Sealing of Wild-Bores Over the years there have been many attempts to seal

or renovate wild-bores. The original method of sealing was to pour as much material into and around the borehole as possible. In some cases this approach appeared to work because there was no surface expres- sion of flow; however, it is possible that subsurface discharges continued to occur.

l h e recent sealing of wild-bores is summarized by Barton and Perkind'). The method was to clean out the bore using air or water and then inflate a packer, preferably where the bore passed through the Blisworth or Lincolnshire Limestones, which should prevent the flow of water. High-pressure grout was then injected under the packer up to the refusal point. Once this was achieved, the packer was retrieved and the remainder of the bore was filled with grout. A rising main was provided for those bores where a compensation flow was required.

In practice, many difficulties were encountered in sealing the bores; for example, the original bore was often

40 0 J.CIWEM,1999, 13, February

The E f f e c t of Borehole Sealing on the Southern Lincolnshire Limestone Catchment

Borehole

damaged, decayed or non-existent. Out of thirty-one repairs attempted, only eight were effected in the Lincolnshire Limestone; of the remainder, fourteen were 'sealed' above one (or both) of the minor aquifers, eight were sealed in the Upper Estuarine deposits, and one was abandoned. Whenever the seal could not be achieved in the Lincolnshire Limestone, the possibility remained of flow into minor aquifers or Fen gravels and other near-surface deposits. These outflows are difficult to id en ti fy.

Compensation flows are supplied by outlets from the rising main at the top of the wild-bores, and the rising mains are usually provided with a valve(+). Sometimes the controlled bore is used to fill a Fen for subsequent surfice irrigation; on other occasions a small flow occurs for compensation purposes.

An important statement in thc Conclusions of the paper by Barton and Perkins is as follows;

'Two problems remain to be resolved. One is the question of the assessment and control of the residual flows to be released from the bores once 'wild' hut now under control. The other concerns the allocation of the ground- water resource conserved which, without re-allocation for abstractive use, will eventually replenish the depleted natural artesian spring-flows along the Fen edge.'

The final comment about the 'spring-flows along the Fen edge' is not strictly correct, but these two issues are important and will be addressed in this paper.

Observations of Curren t State of Wild-Bores In order to understand the different aspects of the

catchment response to the control and sealing of wild bores, a restricted area was examined in detail, and infor- mation about some of the wild-bores in the Billing- borough Fen to Folkingham area is given in Table 1, and their locations are shown in Fig. 4.

Values of recommended minimum flows from Halcrow"), together with preliminary readings of measured wild-bore flows obtaincd using a container and stopwatch, are also given in Table 1. Valve settings are adjusted by certain farmers to provide water required for irrigation; consequently these readings are a sample of wild-bore compensation flows during the dry summer of 1996.

Grid Flow (m3/d) Comments

TF Halcrow Measured on 10/7/96

reference

C o m p u t e r M o d e l f o r S o u t h e r n L i n c o l n s h i r e L i m e s t o n e

121 ~ 438

60

877

I 219 11

, 110

-

S u m m a r y of Model of Southern Lincolnshire Limestone Catchment The Southern Lincolnshire rimestone aquifer was one of the first British aquifers to be studied in detail, and a hydrogeological study is reported by Downing and Williams(6). Many new modelling techniques were then introduced, including aquifers changing between con- fined and unconfined conditions(7), initial conditions@), rapid recharge(9) transmissivity varying with saturated depth, and river-aquifer interaction(IO).

The earlier studies concentrated on the ground- water resources but, due to the increasing importance of the effect of aquifer abstraction on river flows, a further study has been carried out based on integrated modelling of the surface water and groundwater. This re-appraisal of the whole catchment area has led to the quantification of the runoff-recharge(") and more detailed represen- tation of river-aquifer interaction and upwards leakage from the confined regionc2). Another important develop- ment is the improved representation of the wild bore- holes as discussed below.

Hydraulics of Artesian Borehole Discharge In early models, the wild-bores were represented

as springs in which the flow was proportional to a coefficient multiplied by the difference between the groundwater head and the outlet of the wild-bore, as follows:

(1) Q = Coeffs (h - z,,) for h>z,

where Q = discharge (m3/d), Coefls = the coefficient for the particular wild

h bore when represented as a spring,

= elevation of the wild-bore outlet (m). = groundwater head (m), and -

-0

However, considering classical pipe theory, the equation for the head-loss in a pipe is:

j ' l u2 k u7 Head-loss = - + -

Z g d 2 R

Billingborough Fen Pond, L5 Cobshorne Farm, L73 Decoy Farm, L14 Church Farm Pointon, L 15 Connants Farm, L19 Dunsby Fen Farm, L25 Sycamore Farm, L35 Mill Lane, L39

137 343 126 309 137 311 148 315 151 303 163 274 148 211 119 366

130 4

12 14 12

130 90

170

Possible leakage to gravel pack

Seal attempted but abandoned Controlled but external leakage Not sealed

Notes: Halcrow: recommended minimum flows: Table 7.1 of Halcrow (1992)

0 J CIWEM,1999. 13, February 41

D. Johnson. K. R . Rushton and L. M. Tomlinson on

where / = friction factor determined from the Moody graph(I2)

I = length of the pipe (m) u = velocity of the water (m/s) R = acceleration due to gravity (m/s2) d = diameter of the pipe (m) k = head loss coefficient for entry and exit

losses.

The pipe discharge Q equals the velocity multiplied by the cross-sectional area A; therefore the discharge is pro- portional to the square root of the head-loss (or head difference):

Q = u,4 = Coeflp x d(h - a,) (3)

where Coeffp is a coefficient for the particular wild- bore when it is represented as a pipe.

In order to illustrate the effect of the borehole diameter on the head-loss, consider the flow in two boreholes, one of 127 mm dia., the other of 100 mm dia. Using equations (2 and 3) and settingfz 0.2 , k (entry and exit) = 1.5 and a bore length of 60 m; for a discharge of 3.0 Ml/d, the total head-loss (4.72 m) for the 100-mm pipe is almost three times that for the 127-mm pipe (1.66 m).

Inclusion of Wild-Bores in Groundwater Model

bores are included in the groundwater model: The following stages should he followed when wild-

(i) The average wild-bore flows before sealing or control were taken from the mean flows in Uarton and Perkind'? Since the wild-bores are repres- ented at a nodal point on a 1-km square grid, a number of bores are sometimes represented at a

single nodal point, and the totalled flows at the wild-bores simulated in the model are recorded in Table 2.

(ii) For the initial conditions of the groundwater model, the wild-bores are represented as specified flows.

(iii) From this first simulation, head differences between the calculated groundwater head and the specified outflow elevation can bc determined. From equation (3), re-written as,

(4) values of the coefficients for each wild-bore group are calculated, and these values are listed as Coeffp in Table 2. The historical simulation is run with these coefficients and, from wild-bore hydro- graphs, it is possible to see whether the average flow is achieved. Fig. 5 shows a typical model result for Dunsby Fen Farm wild-bore; the average flow (No. 4 of'rable 2) is 1.3 Ml/d.

(iv) To represent the sealing or controlling of the wild- bores, assumed average compensation flows (as specified in the penultimate column of Table 2) are used. These flows are based on the observations during the summer of 1996 and the target flows identified by Halcrow. Due to the uncertainties about these readings, a sensitivity analysis has been carried out to explore the uncertainties associated with compensation flows.

On the basis that equation (3) continues to represent the flows, and that the flows occur under a similar head difference, the coefficient for the controlled boreholes equals the original coefficients multiplied by the ratio of the controlled flow and the unsealed average flow. These flows are listed in Table 2 as Coejji. For Dunsby Fen Farm wild-bore (Fig. 5) the controlled flow is 0.13 Ml/d; this value

Coeflp = Q / (h - z,, )~ '~

Table 2. Wild-bore coefficients used in numerical model to represent uncontrolled and controlled wild-bores

No.

1 2 3 4 5 6 7

9 10 11 12 13 14 15 16 17 18

a

-

Name

Aslackby Fen Cobshorne Farm Rippingale Dunsby Fen Farm Dyke Billingborough Brewery Horbling Springs Billingborough Fen Quadring Bourne North Fen Bourne Maxey House Hereward Cress Beds Bourne South Fen Langtoft School Farm Car Dyke, South Thurlby Marholm Crossing

Ref.

L15, L19 L13, L14 L21 L23, L25 L33 L2, L4, L8, L9 L39, L59 L1, L5, L6, L10, L57 L11, L12, L17 L34, L35 w2 1, w22, w57 w4 W5, 11. 24, 26, 54 W7, W15, W18, W56 w12 L48,L53 W27, W58, W59

Grid Ref TF

150 300 130 310 130 280 160 270 120 220 120 340 120 340 140 340 190 330 150 220 090 200 140 090 100 190 110 190 160 150 170 190 120 170 150 040

Ave Flowcbl Ml/d

3.80 2.0 0.2 1.3 0.3 2.1 0.3 1.6 1.4 1.4 0.7 1 .o 1.5 4.7 0.2 0.4 2.9 1 .o

1227 630 62

41 6 114 689

90 500 467 462 274 446 629

1800 72

144 1544 587

Comp. flow(c) Ml/d

0.1 0.1 0.0 0.13 0.0 0.8 0.18 0.14 0.32 0.6 0.3 0.0 1 .o 0.1 0.2 0.01 0.0 0.0

32 32 0

42 0

262 54 44

107 198 117

0 41 9 38 72 4 0 0

Notes: (a) L15, L19. . . W59, number of wild-bores used in Barton and Perkins (1994). (b) Average flow taken from Barton and Perkins (1994) and other sources. (c) Compensation flow from Halcrow (1992) and supplemented by limited field observations. (d) Coeffp and Coeffc are the coefficients in equation (3) representing uncontrolled and controlled wild-bores respectively.

42 0 J.CIWEM,1999, 13, February

The Effect of Borehole Seal ing on the Southern L inco lnsh i re Limestone Catchment

3.0 -

2.5 -

2.0 -

3 Without control of wild-bores L

h 1.5

e 1 .o

With control of wild-bores I 0.5

0.0

Year Fig. 5. Model results for wild-bore flows at Dunsby Fen farm with historical abstractions

is approximately maintained in all but very dry summers. To illustrate the effect if the wild-bores had not been controlled, the full line of Fig. 5 (representing no control of the wild-bores) is continued until 1996. When a borehole is sealed with no compensation flow, Coeflc is set to zero.

(v) Another significant issue is whether it is possible to identify the effect of controlling or sealing wild-

40

bores from available groundwater head hydro- graphs. Fig. 6 shows the field values of groundwater head at Aslackby, the modelled values assuming no control of wild-bores, and the response when control occurs during 1991-92. The controlling of wild-bores has only a small effect on the ground- water head fluctuation at the current observation sites.

l 1984 1985 I 1986 I 1987 1988 1989 I 1990 I 1991 I1992 I 1993 1994 I 1995

Year

Fig. 6. Field and model results for groundwater heads at Aslackby (based on historical abstractions)

0 J.CIWEM,1999, 13, February 43

D. Johnson, K R. Rushton and L. M . Tomlinson on

Leakage through Overlying Beds

River Slea East Glen West Glen River Gwash

Significance of Controlling Wild-Bores Barton and Perkins(') suggest that the controlling

and sealing of wild-bores saves up to 19.6 Ml/d of resource, and the numerical model as described above is used to test whether this is a valid estimate.

Predictive solutions are obtained with conventional and runoff recharge at the historical values from 1970- 1996 but with abstractions at values which are consistent with the current licences (total abstraction from the Southern Limestone of 80 Ml/d).

'Two predictions are obtained; (i) without any controlling or sealing of the wild-bores, and (ii) with the wild-bores controlled or sealed for the whole simulation period with coefficients as indicated in Table 2. Changes in the aquifer system due to the control of the wild-bores are summarized in Table 3 (note that tolerances of up to 1.0 Ml/d apply to these values); quoted values refer to long-term averages for all the predictive years. Due to controlling or sealing the wild-bores, the model predic- tions are as follows:

Standard scenario(a) Scenario 1 (b) Scenario 2Ic)

5.4 5.0 4.2 3.1 2.5 2.3 2.7 2.1 1.9 2.5 1.2 2.0 1.7 1 .o 1.3

(a) Flows from the wild-bores reduce by an average of 15.4 MVd; the manner in which the differences change with time is shown for part of the predictive period in Fig. 7(a) (where predictive years 7-17 have the same recharge as 197G1986);

(b) The upward leakage through the Overlying Beds increases by 5.4 iMl/d;

(c) Groundwater flows to the River Slea (which is the river catchment to the north of the Glen catch- ments) would increase by 3.1 Ml/d; this occurs because less water is drawn southwards from the Slea catchment to the northern part of the Southern Limestone;

(d) There is an increase in groundwater flows to the West Glen catchment of 2.5 Ml/d (Fig. 7(b)) and an increase in flows in the River Gwash (which is to the west of the West Glen) of 1.7 MVd; and

(e) Groundwater flows to the East Glen increase by 2.7 Ml/d; however, there is a significant variation in the difference in flows with a minimum difference of zero (when flows cease in the East Glen) to a maximum of almost 18 iMl/d (Fig. 7(c)).

Further important information can be gained by examining flows to the east of the wild-bores and abstrac-

(a) Standard Scenario is the standard model (b) Scenario 1, controlled and uncontrolled wild-bore coefficients multiplied by 1.5 (c) Scenario 2, controlled and uncontrolled wild-bore coefficients doubled

tion sites. 'There is poor-quality connate water to the east, therefore flows crossing a north/south line 22 km long, extending from TF 205 125 to TF 205 345, are calcu- lated. The results for predictive years 7-17 are presented in Fig. 8. With control of the wild-bores there would be a small net outflow to the east of 0.18 Ml/d; however, if there had been no control of the wild-bores there would have been a small inflow of poor-quality water of 0.21 Ml/d. This small inflow to pumped boreholes would be of no significance compared with the far higher inflow of good-quality water from the west.

Because detailed field information about uncon- trolled wild-bore flows is not available and there is uncer- tainty about the magnitude of the controlled wild-bore flows, the significance of these uncertainties needs to be explored. This is achieved by carrying out a sensitivity analysis in which the wild-bore coefficients are changed to reflect the uncertainty. Even if the controlled wild- bore coefficients are doubled, the long-term groundwater head at Aslackby is only 0.9 m lower, there is no decrease in flows in the tributaries of the West Glen, the decrease at Shillingthorpe gauging station on the West Glen is 0.8 Ml/d (compared with an average flow 41.1 Ml/d), and a decrease of only 0.1 Ml/d at Manthorpe gauging station on the East Glen. These findings indicate that different wild-bore coefficients would not influence the conclusions about the effect of controlling and sealing the wild-bores.

D i s c u s s i o n a n d Conclusions

An examination of the natural springs and wells in the South Forty Foot drain catchment shows that (a) they originate from minor aquifers in the strata lying above the Lincolnshire Limestone, and (b) provide a significant contribution to the water flowing in channels across the catchment. The artesian flows from wild-bores tapping the underlying Lincolnshire Limestone aquifer provide another important contribution.

Recently, many of these wild-bores have been controlled or sealed. To understand the significance of the controlling and sealing of the wild-bores, they have been represented in existing models of the Southern Lincolnshire Limestone catchment. An important devel- opment is the representation of the wild-bore flows as being proportional to the square root of the difference between the groundwater head and the elevation of the wild-bore outlet. Controlling of the wild-bores, which took place in 1991-92, is represented by decreasing the wild-bore cocfficient.

The mathematical model has been used to estimate the probable long-term effect of controlling or sealing the wild-bores. With an assumed standard abstraction, the average reduction in flow is just over 15 Ml/d; this reduction leads to increases in upward flow through the Overlying Beds and small increases in the flows in the Rivers Slea, East Glen, West Glen and Gwash.

Consideration has also been given to the effect of the wild-bore sealing on possible inflows of connate water from the east. With abstractions at the licensed values, predictive simulations indicate that the worst-case situation would lead to an average inflow over a 27-year period of less than 0.1 Ml/d; if all this water entered

4 4 0 J.CIWEM,1999, 13, February

The Effect o f Borehole Seal ing on the Southern Lincolnshire Limestone Catchment

50 1 (a) Wild Bores

Fig. 7:

7 1 a 1 9 1 10 I 11 I 12 I 13 I 14 I 15 I 16 I 17 ' -50 I

10

5

0

-5

-10

10

5

0

-5

-10

(b) West Glen

7 ' 8 1 9 ' 1 0 1 1 11 I 12 I 13 I 14 I 15 I 16 I 17 '

(c) East Glen

7 1 8 1 9 1 10 I 11 I 12 ' 13 I 14 I 15 I 16 I 17 I Year

Predicted changes in flows (recharge equivalent to 1976-86) due to controlling of wild-bores

0 J CIWEM,1999, 13, February 4 5

0. Johnson, K . R . Rushton and L. M. Tomlinson on

5.0

4.0

3.0

2.0

1 .o

9 0.0

-1 .o

-2.0

-3.0

-4.0

-5.0

\ r \LdI”etflow ‘ ‘ I I \ I

to west N d

Average flow to the east = 0.19 Mlld

7 l 8 ‘ 9 l 10 ‘ 11 I 12 I 13 I 14 I 15 I 16 I 17 ’ Year

Fig. 8. Predicted flows across reference section to east with control of wild-bores

public-supply boreholes, the dilution with the much larger inflows of fresh water from the west would mean that the connate water would pose no threat.

A c k n o w l e d g e m e n t s

T h e study of the borehole sealing in the Southern 1.incolnshire Limestone catchment resulted from a research contract with the Environment Agency (formerly the National Rivers Authority). Further developments have been funded by Anglian Water Services. T h e authors wish to thank members of those organizations for their support during the studies. T h e opinions expressed in this paper are those of the authors and may not necessarily reflect the policies of the Invironment Agency.

R e f e r e n c e s

(1) BARTON, B M J AND PERKINS, M A Controlling the Artesian Boreholes of the South Lincolnshire Limestone J fnst Wat ti Enwr Mangt,

(2) RUSHTDN. K R AND TOMLINSON, L M Total catchment conditions in the 1994 8 (3), 183-186

Southern Lincolnshire Limestone. Submitted to Quart. J. of Engng Geol., 1998.

(3) BLACK SLUICE INTERNAL DRAINAGE BOARD. The Black Sluice lnternal Drainage Board, A Short History 1935-1985,1985.65pp.

(4) HAI CROW. South Lincolnshire Limestone Aquifer: Sealing 01 Wild Boreholes, Final Report. 1993.

(5) HALCROW. South Lincolnshire Limestone Aquifer: Sealing of Wild Boreholes Ecological and Hydrological Assessment. 1992

(6) DOWNING, R. A. AND WILLIAMS, B. P. J. The Groundwater Hydrology of the Lincolnshire Limestone Wafer Resources Board, Publ. No. 9, 1969.

(7) RUSHION, K. R. AND WEDDERBURN, L z. Aquilers changing between the unconfined and confined state. Ground Waf., 1971,9, 30-39

(8) RUSHTON, K. R AND WEDDERBURN, L. A. Starling conditions for aquiler simulations. Ground Wat.. 1 9 7 3 , 2 l l , 37-42

(9) Fox, I . A. AND RUSHTON, K R Rapid recharge in a limestone aquifer. Ground Wat., 1976,14, 21-27.

(10) RUSHTON, K. R., SMITH, E. J. AND TOMLINSDN, L. M. An improved under- standing of flow in a limestone aquiler using field evidence and mathematical models. d fnsfn. Waf. Eng. Sci., 1982, 36, 369-387.

(11) BRADBURY, C. G . AND RUSHTON. K. R. Estimating runoff-recharge in the South Lincolnshire Limestone Catchment, UK. (Submitted to J. Hydro/.). 1998

(12) DAUGHERTY, R. L.. FRANZINI, J . B. AND FINNEMORE, E. J. Nuid Mechanics with Engineering Applications. Mc-Graw Hill, 1989.

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