Midland Valley

Coverage of baseline sampling in the Midland Valley against a simplified 1: 625 000 scale geology map

A total of 62 groundwater chemistry analyses were used to interpret the groundwater chemistry of the Carboniferous aquifers of the Midland Valley. Of these, 36 were from new groundwater samples collected for the Baseline Scotland project in September and December 2008 and one from a sample collected during an earlier Baseline Scotland sampling programme in the Borders. These were augmented with a further 25 samples collected during separate projects since 2001, which fill in many of the spatial gaps in sample collection across the study area.

The sites for the samples used in the study were chosen to be representative of groundwater in its natural state from the Carboniferous aquifers in the Midland Valley. However, given the history of mining in this region, a number of the sampling sites were known to be affected by mining, and others may also have been affected. A small number of samples were deliberately targeted at coal mine workings.

Five aquifer groups have been identified: four chronostratigraphic groups, which in decreasing order of age are the Inverclyde, Strathclyde, Clackmannan and Coal Measures groups; and a fifth group incorporating waters known to be derived directly from former coal mines.

Main findings

The collection and interpretation of new groundwater chemistry data for the Carboniferous aquifers of the Midland Valley has led to the following conclusions:

  • Most of the groundwater samples have a near-neutral pH (10th to 90th percentile range of 6.40–7.85). There is apparent little difference in pH between the different aquifer groups. This indicates that the acid buffering capacity of all of the aquifers is high, most probably controlled by dissolution of carbonate minerals.
  • Alkalinity is variable, but often high. Given the near-neutral pH of most of the groundwaters, the main species contributing to the alkalinity is likely to be HCO3.The 10th-90th percentile range in bicarbonate concentrations is 132-510 mg/L The highest median bicarbonate values are for the Coal Measures Group and mine waters, although some high concentrations also occur in the Clackmannan Group.
  • Groundwater types encompass a large range from Ca-HCO3, through Ca-Mg-HCO3, through SO4-rich types, Na-HCO3, and (rarely) Na-Cl. The groundwaters have variable calcite saturation indices but in many cases are close to saturation, indicating both the presence of and reaction with carbonate minerals in the aquifers.
  • There is a large range in concentrations of the major anions. Concentrations of Cl are typically high, but also highly variable with a 10th–90th percentile range of 7.2–123 mg/L. Concentrations of SO4 span some five orders of magnitude with a 10th–90th percentile range of 4.7–179 mg/L. The overwhelmingly dominant processes controlling this large range are likely to be oxidation of sulphide minerals, particularly in mining-impacted waters; and sulphate reduction in the most reducing conditions. Low concentrations (<5 mg/L SO4) are usually associated with low NO3-N concentrations and often high NH4-N; all samples recorded in the field as having a smell of H2S had SO4concentrations <5 mg/L.
  • There is large spatial variability in concentrations of the major cations. The highest concentrations of Ca and Mg are in mine waters and some Coal Measures Group groundwater, probably due to reaction of carbonate minerals, and clays, which could have been enhanced by the generation of acid during the oxidation of iron sulphide. Concentrations of Na are also often high, particularly in groundwaters from the Coal Measures and Clackmannan groups and mine waters. Concentrations of K are also correspondingly high. The high concentrations of dissolved cations are likely to reflect the effects of extreme water-rock interaction, in some cases (particularly deep boreholes) involving long residence times in the aquifers concerned. Sodium is probably largely sourced from dissolution of silicate minerals and ion-exchange reactions, the latter induced by young shallow groundwater mixing with older saline water.
  • Concentrations of Fe and Mn show large ranges, consistent with the large variations in redox status in the aquifers. The highest Fe and Mn concentrations are in groundwater from the Coal Measures Group and Clackmannan Group. Both elements are found at highest concentrations in groundwaters with low dissolved oxygen concentrations, in agreement with the recent findings of Homoncik et al. 2010. [link this to the paper download on the Mn page]
  • Many of the collected groundwaters are reducing (anoxic), with 37% of determined dissolved oxygen values less than or equal to 1 mg/L and 60% of determined Eh values less than 250 mV. Groundwaters from the Clackmannan Group and Coal Measures groups are the most reducing (excluding the mine discharges). Field sampling showed evidence that some of the groundwater sources from the Clackmannan and Coal Measures groups contained dissolved sulphide, indicating the presence of strongly reducing conditions in some parts of these aquifers. Detectable dissolved methane was measured in a number of the samples. Samples from the Strathclyde and Inverclyde groups show the highest dissolved oxygen concentrations. High concentrations of dissolved organic carbon (DOC) (up to 70.8 mg/L) in a few samples are also indicative of the reducing – often strongly reducing – nature of some of the groundwaters.
  • Nitrate concentrations are highly variable with little relationship with land use, showing the strong control that redox conditions have on groundwater chemistry, and indicating that denitrification – which occurs under reducing conditions – has been important in many of the groundwaters. Nitrite is detectable in a few low-NO3 groundwaters and, as an intermediate reaction product, is an additional indication of denitrification.
  • The highest concentrations of phosphorous are in the Coal Measures and Clackmannan groups, with sources likely to include dissolution of phosphate minerals, desorption from iron oxides under reducing conditions and degradation of organic matter. Agricultural pollution may be contributory in some cases, but some of the outlier P concentrations occur in the more reducing (and mine-impacted) groundwaters, which would be consistent with a natural (mineral, organic matter) origin.
  • Dissolved gases. Dissolved carbon dioxide (CO2) was measured on selected groundwaters and the results indicate that pCO2 is often high, controlling carbonate equilibrium. Dissolved methane (CH4) was present at 10 μg/L or less in approximately half the samples measured. Such values are typical of those found in major aquifers such as Cretaceous Chalk and the Permo-Triassic Sherwood Sandstone Group. The remainder of the samples range up to ca. 10 mg/L. Waters with dissolved CH4 concentrations higher than 1.5 mg/L can in principle give rise to explosive atmospheres in confined situations such as buildings and excavations.

There are some distinct differences in groundwater chemistry between the aquifer groups. The most mineralised groundwaters (excluding mine discharges) are from the Coal Measures Group, which also have the lowest dissolved oxygen concentrations, At least some of the groundwaters identified as being from the Coal Measures Group are likely to have been impacted in some way by coal mining, The least mineralised groundwaters were from the Inverclyde and Strathclyde groups.

Mining

Mining activity has had a major impact on groundwater quality in the region. This is particularly the case for the samples collected directly from discharges (pumped or gravity flows) from abandoned coal mines. Six samples of groundwater were sourced directly from abandoned coal mines, and show a wide range of chemical types. The mine-discharges are typical strongly mineralised, with high electrical conductivity (SEC) values and particularly high concentrations of HCO3, Ca, SO4, Fe and Mn. The waters are generally low in dissolved oxygen and show the evidence of pyrite oxidation within the mines. The pH values are generally well-buffered and alkalinity is high, indicating significant reaction with carbonate material in the aquifers.

Five of the Baseline samples – two from the Clackmannan Group and three from the Coal Measures Group – were collected from sources known or strongly suspected to intercept abandoned mine workings. Although none of these sources was specifically abstracting mine waters, it is likely that at least some of the groundwater pumped from them derives from the mine workings. These five samples tend to be more mineralised than samples which are thought unlikely to be affected by mining, although generally not as mineralised as the 'Mine' waters. This is particularly the case for concentrations of HCO3, K, SO4, Fe and Mn, and for the electrical conductivity of the groundwater. Average Ca concentrations in these five samples are as high as those in the 'Mine' waters. Average concentrations of Cl, Mg and Na in these samples are similar to those in samples thought unlikely to be impacted by mining.

Interpretations in terms of groundwater flow

The hydrochemical data and information on groundwater residence times can help give an insight into groundwater flow in aquifers across the Midland Valley. Many of the groundwater samples from the studied aquifers are from deep boreholes, where groundwater is often present under confined conditions and within multilayered aquifers. Groundwater flow is dominated by flow within fractures. Analysis of dissolved gases and stable isotopes indicate that groundwaters contain a high proportion of relatively old water, recharged more than 35 years ago, and a significant proportion of water recharged more than 60 years ago. However, there is little evidence of the existence of palaeowater (older than 10 000 years).

Many of the groundwaters show evidence of having been impacted by ion-exchange reactions (Na for Ca exchange), which suggests that young shallow groundwater has mixed with older, more mineralised water.

Data

Summary statistics

The tables below provide a statistical summary of the natural variation in groundwater chemistry in all sedimentary Carboniferous aquifers across the Midland Valley (the Inverclyde, Strathclyde, Clackmannan and Coal Measures groups), except 'mine' waters. Data between the 10th and 90th percentiles for each element or ion are presented, which allows the influence of outliers to be minimised. We would expect new data to plot within this range 80% of the time.

Coal Measures Group

Groundwaters from the Coal Measures Group are generally of bicarbonate type, with cations either dominated by Na or with no dominant cation. Average alkalinity values are the highest of all the hydrogeological units sampled, including the 'Mine' waters. There is a large range in SEC values and the median value is the highest for all the groups except 'Mine' waters. The waters are generally anoxic and slightly acidic to near-neutral. The groundwaters typically have moderate concentrations of the major cations Ca, Mg, Cl and SO4, and high cncentrations of Na relative to the other aquifer groups. In most cases the calcite saturation index showed the groundwaters were close to saturation. There is a large range in Fe and Mn concentrations but concentrations are usually high, with the highest average of any group except the 'Mine' waters.

Clackmannan Group

Groundwaters from the Clackmannan Group are generally either of Ca-Mg-HCO3 type Na or have no dominant anion; a few show a cationic dominance of Na. The groundwaters typically have high HCO3 concentrations, a near-neutral pH, and generally low dissolved oxygen. SEC values show a wide range but a moderate average. The waters generally have moderate to high concentrations of the major cations Ca, Mg, Ba and Cl. Concentrations of SO4 were the highest of all the hydrogeological units except 'Mine' waters. Most of the samples were significantly undersaturated with respect to calcite. Concentrations of Fe and Mn were moderate to high, with variability due to the redox conditions in the aquifer.

Inverclyde Group

Most of the groundwaters from the Inverclyde Group were of Ca-HCO3 type. They typically have moderate HCO3 with a near-neutral pH. Dissolved oxygen concentrations are generally low but most of the waters are not anoxic. SEC values are typically moderate. The groundwaters typically have relatively low concentrations of the cations Na, Cl and SO4 and moderate concentrations of Ca and Mg. In most cases the calcite saturation index showed the groundwaters were close to saturation. Concentrations of Fe and Mn are typically relatively low, reflecting the generally oxic nature of the groundwaters from this group.

Strathclyde Group

Most of the groundwaters sampled from the Strathclyde Group were of Ca-Mg-HCO¬3. Some samples are more dominated by Na with no anionic dominance. The groundwaters typically have moderate HCO3 concentrations with near-neutral pH. Dissolved oxygen concentrations are usually low but most of the waters are not anoxic. SEC values are generally moderate to high. The waters typically have moderate concentrations of the cations Ca, SO4, Na, Cl and Mg. In about half of the samples the calcite saturation index showed the groundwaters were close to saturation or saturated with respect to calcite; the other half were undersaturated. Iron concentrations are relatively low on average but show a wide range. Mn concentrations are typically moderate.

Maps of regional variation in selected ion concentrations

Baseline Report

Ó Dochartaigh, B É, Smedley, P L, MacDonald, A M, Darling, W G and Homoncik, S. 2011. Baseline Scotland: groundwater chemistry of the Carboniferous sedimentary aquifers of the Midland Valley. British Geological Survey Open Report, OR/11/021.