Moray Firth

Coverage of baseline sampling in the Moray Firth against a simplified 1: 625 000 scale geology map.

A total of 39 groundwater samples have been interpreted to investigate the groundwater chemistry of the Old Red Sandstone aquifers in the Moray Firth area . Of these, 17 were collected in 2007 specifically for the Baseline Scotland project. These were augmented with a further 22 samples collected during separate BGS projects since 2001. The sites were chosen so that the data would be representative of groundwater across the Old Red Sandstone aquifers in the area.

Three main aquifer units were sampled: the Lower, Middle and Upper Old Red Sandstones in the Moray Firth area.

Main findings

The collection and interpretation of new groundwater chemistry data for the Old Red Sandstone aquifers in the Moray Firth area has led to the following conclusions:

  • Groundwater in the Old Red Sandstone aquifers of the Moray Firth is generally moderately mineralised, with a median SEC of 469 μS/cm (interquartile range 341–591 μS/cm). The major ion chemistry appears to be dominated by the dissolution of carbonate cements within the aquifer and overlying deposits, and the variable influence of seawater (either directly as saline intrusion or as aerosols).
  • Groundwater pH values are variable (5.33–8.06), but the median pH value is near-neutral (7.31). In general the pH of the Lower and Middle Old Red Sandstone aquifers is slightly more acidic, usually less than 7.0.
  • Concentrations of the major cations. The dominant cation is Ca, with a median concentration of 54.8 mg/L (interquartile range 42.5–74 mg/L). Mg and K concentrations are generally low (median 4.21 and 3.7 mg/L, respectively). The median Na is 20.6 mg/L (interquartile range 12.6–28 mg/L); however, a few samples have been affected by the proximity to the sea and have much higher concentrations (95th percentile 68 mg/L, and maximum concentration 153 mg/L).
  • Concentrations of the major anions. The dominant anion is bicarbonate, with a median concentration of 183 mg/L (interquartile range 183–230 mg/L). Around one third of the samples are saturated with respect to calcite. Sulphate concentrations are generally low (median 14.6 mg/L, interquartile range 7.7–36.4 mg/L), although higher concentrations are encountered in samples affected by seawater, and/or possibly by gypsum bands within the aquifer. Chloride concentrations follow broadly the same distribution as Na and have a median of 38.8 mg/L and interquartile range of 19.5–49.5 mg/L; the same few samples show high Cl concentrations as do Na.
  • There is a large range in redox conditions across the aquifer. Sub-oxic or mildy reducing groundwaters (where dissolved oxygen (DO) concentrations are <1 mg/L) are prevalent across much of the Upper Old Red Sandstone, and parts of the Middle Old Red Sandstone. These reducing conditions may reflect the presence of low permeability layers (often marine in origin) within the thick superficial deposits overlying the Old Red Sandstone aquifer.
  • Concentrations of minor and trace elements are dominated by the redox conditions. Concentrations of Fe and Mn are relatively high, with median concentrations of 38 and 43 μg/L respectively, and 75th percentile values of 354 and 227 μg/L respectively. Concentrations are highest in the Upper Old Red sandstone outcrop, and parts of the Middle Old Red Sandstone, reflecting the reducing nature of the groundwaters.
  • Nitrate concentrations are variable across the aquifer units, although median concentrations are low (1.45 mg/L TON-N or less in each aquifer). The prevalence of low oxygen conditions in the sampled groundwaters has led to denitrification, which means the relationship between land use and nitrate concentrations is less obvious than for other parts of Scotland (MacDonald et al. 2005). However, there is a clear relationship between nitrate concentrations and the Nitrate Vulnerable Zone (NVZ) that covers much of the study area, with the seven highest groundwater nitrate concentrations, ranging from 5.98 to 22.1 mg/L TON-N, all from samples taken within the NVZ. The highest median concentrations were from samples collected on land known to be used for dairy, pig or poultry farming.
  • Concentrations of phosphorous range from less than detection limit up to 172 μg P/L, with an overall median of 36 μg P/L, which is in the eutrophic range for surface waters. Concentrations are generally low in the western part of the study area, and an observed relationship with the spatial pattern of F suggests that both elements may be in part derived from the dissolution of phosphate minerals, such as apatite, from the aquifer rocks. Concentrations in the eastern part of the study area are generally higher, usually in the mesotropic or eutrophic range for surface waters. The higher values may be related to land use, with P inputs from agricultural activity.

Interpretations in terms of groundwater flow

The information on hydrochemistry and groundwater residence times can help give an insight into groundwater flow in the Old Red Sandstone aquifers in the Moray Firth area. There is no evidence of palaeowater in the samples taken from the area; most of the groundwater are interpreted as being less than 50 years old using the SF6 data. The reducing nature of the groundwaters make it difficult to interpret the CFC data (since both CFC-11 and CFC-12 have been reduced) and get a reliable estimate of whether flow is predominantly fracture dominated or piston intergranular flow. Two samples that were not reducing appear to show piston flow; however, groundwater chemistry (in particularly nitrate) show no significant depth correlations, implying that groundwater is well mixed. A possible explanation is that piston flow occurs in the thick overlying superficial deposits, but within the sandstone fracture flow may dominate.

Carbon isotope evidence indicates the significant role of the nature of the superficial cover in impacting the groundwater chemistry, showing differences between groundwaters recharged through glaciofluvial deposits, which are likely to derive bicarbonate from silicate hydrolysis, and those recharged through marine or beach deposits, in which bicarbonate is likely to derive from dissolution of shell carbonate.

Data

Summary statistics

The table and plot below provide a statistical summary of the natural variation in groundwater chemistry in the Old Red Sandstones aquifers across the Moray Firth area. 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, with the exception of NO3-N and P, where the influence of anthropogenic activity is likely to have distorted baseline conditions throughout much of the area.

Lower Old Red Sandstone

The four Lower Old Red Sandstone groundwater samples showed a range in cation dominance from Ca to Na-K. All but one are dominated by HCO3 anions, with the remaining sample strongly dominated by Cl. Dissolved oxygen and redox (Eh) values are only available for two of the samples, which were oxic. Three of the four samples had below-neutral pH values, and the median was 6.34, lower than the median for the whole Old Red Sandstone dataset. Concentrations of most major ions are lower on average than in Middle and Upper Old Red Sandstone groundwaters, except Mg which is similar. Relatively low Ca concentrations and pH values probably result from a lack of carbonate minerals in the aquifer or overlying superficial deposits, which is also reflected in the lower calcite saturation values for these groundwaters (all but one of the samples was strongly undersaturated with respect to calcite). Iron concentrations are typically low, while manganese concentrations show a larger range, with a low media value.

Middle Old Red Sandstone

Groundwaters from the Middle Old Red Sandstone aquifer show an overall cationic dominance by Ca, but a relatively wide spread, trending towards no dominant cation. Their anionic distribution is more restricted, strongly dominated by HCO-3, apart from a single Cl-dominated water. They show very little SO4 influence. Most of the samples are undersaturated with respect to calcite, but less strongly so than the Lower Old Red Sandstone waters.

Dissolved oxygen and Eh measurements are available for only seven of the fourteen Middle Old Red Sandstone samples, of which four are oxic and three are anoxic. The groundwaters have a higher median pH than the Lower Old Red Sandstone waters, and the highest median HCO3 concentration of the three aquifers. SEC and median Ca, Na, K and Cl concentrations are in between the average values for the Lower and the Upper Old Red Sandstone groundwaters. Concentrations of SO4 are, as for the Lower Old Red Sandstone, relatively low, while magnesium concentrations are comparable to the other two aquifers.

Iron and Mn concentrations are highly variable, from low (less than the detection limit for Fe) to high, although both show low median concentrations. The main control on the presence of high Fe and Mn is likely to be the presence of anoxic conditions within the aquifer (Homoncik et al. 2010).

Upper Old Red Sandstone

Groundwaters from the Upper Old Red Sandstone aquifer typically show very similar cation distribution, dominated by Ca in all but one case, and closely grouped. They show a wider anionic range, from HCO3 towards Cl and SO4. Most of the samples for which DO and Eh measurements are available are anoxic; only three of fourteen samples showed clearly oxic conditions. The groundwaters have the highest median pH value of the three aquifers, and a slightly lower median HCO3 concentration than the Middle Old Red Sandstone waters. The median SEC value and median concentrations of all of the major ions except Mg are also the highest of the three aquifers. SO4 concentrations in particular are noticeably higher than the other two aquifers. Iron and Mn concentrations are also highly variable, from low (less than the detection limit for Fe) to very high. The median Fe concentration is not especially high, but the median Mn concentration is more than twice the drinking water limit. The high concentrations of both of these parameters are linked to the prevalence of anoxic conditions across much of the aquifer (Homoncik et al. 2010).

Maps of regional variation in selected ion concentrations

Reports

Ó Dochartaigh, B É, Smedley, P L, MacDonald, A M, and Darling, W G. 2010. Baseline Scotland: groundwater chemistry of the Old Red Sandstone aquifers of the Moray Firth area. British Geological Survey Open Report, OR/10/031.

MacDonald, A M, Griffiths, K J, Ó Dochartaigh, B É, Lilly, A and Chilton P J. 2005. An overview of groundwater in Scotland. Scottish Journal of Geology, 41; 1, 3–11.