Assignments of Groundwater Flow Modelling in Dumfries Basin New Site Construction

Introduction:

At southwest of Scotland and in the lower part of catchment of River Nith we can locate Dumfries Basin. The basin can be described to deep, partly fault bounded, outlier of Permian sandstone and breccia, with a partial superficial cover comprising a variety of lithologies which range from gravel to clay. From these characteristics the Dumfries Basin Aquifer strata can be defined. The water for domestic, public including private, agriculture and industrial usage are drawn from this Dumfries Basin Aquifer.

From one of the report it read that the Dumfries basin aquifer and river Nith including tributaries have a complex behaviour over the basin. This calls the interest in a hydrogeologist to study Dumfries Basin Aquifer hydrogeology.

The aim of the report is to detail about the best possible solution for the exaction work 5m below the ground level. for a new site area of about 0.4km2 within the basin. This report will answer and aims to include the following. Firstly, Groundwater Level restriction on excavation process and relevant aspects. Secondly impacts on water budgets. Thirdly, the groundwater flow around the site.

The aims of the project are accomplished through the numerical modelling of groundwater flow that Ground waterflow Vistas 6 software.

Figure 1: Dumfries Basin Map (OS Map, 2004)

Dumfries’s basin identification:

Solid Geology:

The sequence Dumfries Basin bedrock aquifer consists of Doweel Breccia and Locharbriggs Sandstone formations. In terms of age these formations  are Permian. In the western part of basin consists of Doweel Breccia which are prominently sedimentary breccia bedded with sandstone. This formation is seen in the artesian direction directing towards the centre of basin. Further in the northern and eastern parts, the above formations interlinks with formation of Locharbriggs Sandstone. (Jackson et al, 2004)

The Permian sequence unconformably overlies on a steeply dipping succession of grey, fine-grained, and mudstone. This is interfered by the Criffel-Dalbeattie granodiorite which is in south-west region of basin. The Permian basin-fill sequence has thickness ranging from 1.1Km to1.4 km. This is inferred by air-born gravity data model. The basin is described to be fault bounded along its western margin and the north-east regions. (Jackson et al, 2004)

Figure: Bedrock Geology

Quaternary geology

The superficial geology of the Dumfries Basin is widely dominated by glacial deposits, which includes granular and cohesive. Ice moulds originates in Southern uplands along the south-east of Dumfries.

Basal deposit is found to be present lying on Permian strata at north-west region of Dumfries. Fine sand, silt and clay, are found in the south-east of Locharbriggs with dropstones. The Moraine deposits are formed by Nith glaciers and they are composed of folded and sheared glaciolacustrine sand and silt, which usually exceed 30m thichness. It is found to be between Locaharbriggs and Cargenbridge in arc shape. Along the south of Dumfries, extensive deposits of bedded sand, clay and silt are found due to delegation and rise in sea level. These deposits are presently dissected terraces lying at an elevation of 10 to 15 m AOD, due to isostatic force (McMillan, 2002). Marine clays are found be overlying by sand bed along  south of Cargenholm. Estuarine and tidal flat fine-grained sediments are lying are found on flat grounds which has up to 10m AOD throughout the coast. Large amount of peat basins are developed locally due to Lochar Moss and Kirkconnell Flow. From glacial sediment, Alluvial sediments of the Dumfries Basin are comprising of silt, sand and gravel. These are recently occupied through the floodplain valley and also developed due to lowest terraces by River Nith and its tributaries. These comprise gravel, sand and silt reworked from the glacial sediments. (Jackson et al, 2004)

Figure: Superficial Geology

Hydrology:

The Dumfries Basin is a low-lying flat land with rivers found to be in flatter terraces. The mounded sands and peat infilled hollows are the important aspects which are controlling factors of basin geology. The figure shows the hydrology and main rivers flowing through Dumfries basin. The River Nith is a vital watercourse draining along the Dumfries basin. This river is also attached to convergent and sub-parallel tributaries while along the basin. This river is lies more on the right bank. Ultimately all the water is drained towards Solway Firth. (Jackson et al, 2004)

Figure: Ground surface elevation from 50m DTM(Jackson et al, 2004)

Hyrogeology:

Recharge:

Recharge depends on landuse, rainfall and superficial geology. The figure below shows the potential recharge of Dumfries basin. To the North half of basin there is a good and moderate recharge potential. This shows that the most superficial deposits have good favourable permeability level. Whereas, towards south the basin lack recharge potential which is termed to be moderate or low in most of the scenarios. (Jackson et al, 2004)

Aquifer Properties:

Based on high values of transmissivity are found through Terregles boreholes analysis. These showed active fracture flow exceeding 100m depths. This resulted in high transmissivity of 94 m2day-1. There are low transmissivities calculated within the basin which are due to lower borehole depths of 40 m. In south west region of the basin, estimates of Transmissivity is found to be 10 m2day-1. Due to thin sandstone aquifer along the south-east results in lower aquifer transmissivities. (Jackson et al, 2004)

Groundwater Heads:

Groundwater flow pattern is as shown in the figure below. It is seen that groundwater flows towards river Nith and Lochar water of the central basin. Due to high ridge points like in southern Dumfries, in Permian water levels are above superficial rock strata. Surface topography can be understood by the piezometric surface depth. This indicates the occurrences of confined conditions and found to be 15m of unsaturated rock lying down. When the surface level is equal or below to 10m and the main aquifer is covered with silty clay then artesian flow occurs. On the other hand, horizons of sandstones which are separated from breccia including induvial fractures have different heads. (Jackson et al, 2004)

Recharge Distribution by Jackson, Hughes, and O Dochartaigh (2004)

Model Development:

Model Building:

The software that is used is Groundwater Vistas Version 6 (GV6). The basis for this model is developed based on the concepts shown below figure. Figure 2 shows the flow chart how the model was built. The screen of the entire process is being attached as screen shot.

 

Running Model:

Data and Model Building:

Note all units in metre per day are being converted to metre per second.

Calibration results:

CONTOUR LINES DISPLAY OF MODFLOW RESULTS

MAGIFIED WINDOW OF MODEL RESULTS ……………………………………………………………………. 39

ERRORS OBTAINED SCREENSHOT

MASS BALANCE

MODFLOW MASS BALANCE WINDOW

GRAPH FOR MASS BALANCE SUMMARY OF THE MODEL

SECTION PROFILE OF COLUMN 8 SHOWS HEAD-DISTANCE RELATIONSHIP

DEM ELEVATION MAP OF DUMFRIES BASIN USING ARCMAP

ASSIGNING SIX ZONES OF HYDRAULIC CONDUCTIVITY FOR LAYER 2

RECHARGE ZONES OF LAYER 1

GRAPH OF OBSERVED VS. COMPUTED TARGET VALUES

SENSITIVITY ANALYSIS GRAPH

SENEITIVITY ANALYSIS GRAPH INFORMATION

TARGET STATISTICS WINDOW

Transient Model:

References:

Ordnance Survey topographic material with the permission of Ordnance Survey on behalf of The Controller of Her Majesty’s Stationery Office, © Crown copyright. Licence Number: 100017897 [2004].

Jackson et al 2004, Jackson CR, Hughes AG and O Dochartaigh BE; Preliminary numerical modelling of groundwater flow in the Dumfries basin;