National methane baseline survey: southern England

In the south of England, 251 sites have been sampled for methane from a number of different aquifers. The two main aquifers are the Chalk Group and the Lower Greensand Group.

geological map of southern england — Geology of southern England. Contains Ordnance Survey data © Crown copyright and database rights 2015. BGS © UKRI.

Chalk

The Chalk is a principal aquifer in southern England, used extensively for public water supply. It has a high porosity but low matrix permeability, and groundwater flows through a well-developed interconnected network of fractures. The Chalk crops out in the Chilterns, the North and South Downs, Salisbury Plain, the Wessex Downs and on the south coast. It reaches a maximum thickness of 200 m.

Lower Greensand

The Lower Greensand aquifer consists of sands and sandstones, and groundwater flow is mainly through the pore spaces in the rock where the sand is unconsolidated. Groundwater from this aquifer is also used for public water supply. The Lower Greensand crops out around the edge of the Weald and reaches a maximum thickness of 220 m.

Shale units

A NSTA report on the Jurassic shales of the Weald Basin reported that there is unlikely to be any shale gas potential in this region, but there could be shale oil reserves, although how much of this could be recovered is unknown. The shale units present in this region are:

The Kimmeridge Clay and Amptill Clay formations consist of mainly black shales; the Kellaways, Oxford Clay and Osgodby formations consist of mudstones, clays and siltstones.

Results

These summaries are based on the results collected from single visits to each site for the purpose of the methane baseline project. The data are summarised for the Southern England region as a whole, and also for individual aquifers, where enough data are available.

The blue boxes highlight where summary data are available for baseline groundwater quality in the different aquifers present in this region, and link to the summaries for each region.

bar chart showing methane concentration for different aquifers in southern England. — Methane concentrations chart for southern England. BGS © UKRI.

Area	Concentration	(mg/l)		No. samples
	Minimum	Median	Maximum
Southern England	<0.00005	0.0013	4.72	251
Chalk	<0.00005	0.0005	0.083	137
Greensand	<0.00005	0.0049	0.105	54

_{Methane samples and concentrations in Southern England. Note: values less than the analytical detection limit have been converted to half the detection limit for the purposes of this statistical summary and graph, but the relevant detection limit is quoted in the table. Thames Water and the Environment Agency are acknowledged for additional sample collection.}

Baseline groundwater quality data

This data was collected as part of a collaborative project between BGS and the Environment Agency to investigate the baseline quality of groundwater in major UK aquifers. The UK baseline study results page provides downloads of all the reports produced by the project.

Chalk of the North Downs, Kent and east Surrey

The Chalk in this region generally contains water of high quality, the main properties being determined by natural reactions between rain water and the carbonate rock (HCO₃).

Groundwater is oxidising in the unconfined areas, but becomes rapidly reducing once confined by the Palaeogene deposits. The confined groundwaters are typically fresh and of pristine pre-industrial quality but may contain residual concentrations of chloride (Cl) from older water at depth. Some unconfined Chalk groundwaters show the effects of pollution from industrial, domestic and agricultural sources.

Analysis	Concentration	(mg/l)		No. samples
	Minimum	Median	Maximum
Ca	15.7	116	399	132
Mg	1.1	2.8	689	132
Na	6.2	15	5820	132
K	<0.4	1.92	175	132
Cl	10	26	10700	134
SO₄	<10	21.4	1140	133
HCO₃	162	274	493	108
NO₃.N	<0.01	6.2	28.2	133

_{Baseline quality data for the Chalk in the North Downs, Kent and east Surrey.}

Lower Greensand

The groundwater in the Lower Greensand is fresher than water from most other aquifers in the UK, mainly due to the relative purity of the sands and often short residence times. The low salinity of the groundwater even in the deep confined parts of the aquifer shows that it has been well flushed of any older water.

Where the aquifer is confined, the groundwater is more reducing and has higher iron concentrations.

box and whiskers plot for Lower Greensand — Box and whiskers plot for the Chalk in Dorset. BGS © UKRI.

Analysis	Concentration	(mg/l)		No. samples
	Minimum	Median	Maximum
Ca	37	106	143	59
Mg	1.59	2.5	19.4	59
Na	6	11.2	155	59
K	0.5	1.7	7	33
Cl	14	21	223	59
SO₄	<5	13.6	52.2	60
HCO₃	107	271	324	33
NO₃.N	<0.1	6.6	12	59

_{Baseline quality data for the Lower Greensand in southern England.}

Chalk in Dorset

The Dorset Chalk contains groundwater predominately of very high quality. However, much of the area is farmed, the long-term effects of which have affected the natural groundwater quality. The main observed effect of agricultural activity has been the increase in nitrate (NO₃.N) and potassium (K) over several decades. Apart from these examples, there is no evidence of widespread contamination from urban and rural wastes or other sources of pollution.

Oxidising conditions found in the unconfined aquifer rapidly give way to a reducing environment as the aquifer becomes confined beneath Palaeogene deposits.

Analysis	Concentration	(mg/l)		No. samples
	Minimum	Median	Maximum
Ca	37	106	143	59
Mg	1.59	2.5	19.4	59
Na	6	11.2	155	59
K	0.5	1.7	7	33
Cl	14	21	223	59
SO₄	<5	13.6	52.2	60
HCO₃	107	271	324	33
NO₃.N	<0.1	6.6	12	59

_{Baseline quality data for the Lower Greensand in southern England.}

Palaeogene

The water quality of oxidising Palaeogene groundwaters of near-neutral pH is, in the most part, good. High iron and manganese as well as slightly acidic conditions are characteristics of the Palaeogene groundwaters. In these more acidic groundwaters, mobilisation of some trace metals can occur.

Some unconfined waters in the Palaeogene are influenced by agricultural activities, showing elevated nitrate (NO₃.N) concentrations.

The hydrochemical properties of the groundwaters in the Palaeogene aquifers are primarily determined by natural reactions between recharge water and the Palaeogene host rocks.

Box and whiskers plot for the Palaeogene in southern England. BGS © UKRI.

Analysis	Concentration	(mg/l)		No. samples
	Minimum	Median	Maximum
Ca	5.2	46.1	128	26
Mg	1.72	5.35	15.7	26
Na	11.4	19	111	26
K	0.9	2.4	15.5	26
Cl	17.4	33.3	108	26
SO₄	2.7	27.4	91.8	26
HCO₃	5	153	315	25
NO₃.N	<0.01	0.0244	34.8	26

_{Baseline quality data for the Palaeogene in southern England.}

Contact

Please contact BGS Enquiries for more information.