Is the Remediation at Parys Mountain Successfully Reducing Acid Mine Drainage ?

Metal ion concentrations and acidity were used as indicators of acid mine drainage (AMD) at Parys Mountain, a large abandoned copper mine on Anglesey, Wales. Water samples were collected in two sessions and taken from a linear stream flowing from the northern side of the mine, and a stream flowing from the south side of the mine that has two settling ponds and long stretches of wetland along its path. pH measurements were taken to measure acidity levels and metal ions (Fe, Al, Zn, Cu, Mn, and Pb) were quantified by inductively coupled plasma (ICP-OES) spectrometry. The pH values at the settling ponds and northern stream were between 2 and 3 while the wetlands had pH values of 5 6 implying that it was the wetlands that reduced acidity, and not the distance downstream. Both streams showed a reduction in concentrations of all elements with distance downstream. The decrease was linear for the northern stream and exponential for the southern stream, suggesting that the reed beds and settling ponds were successful at removing metal ions; potentially, through slower flow rates allowing more time for redox reactions to occur, thus precipitating metal hydroxides and pure metals and removing them from solution. In November, the northern stream had substantially higher concentrations of Fe, Al, Zn, Cu, and Mn, but not Pb (126, 34.0, 29.1, 14.6, 10.4, and 0.064 mg/L respectively) in solution when compared to the southern stream, which had concentrations of 10.2, 12.2, 11.9, 2.43, 6.11, and 0.706 mg/L for Fe, Al, Zn, Cu, Mn, and Pb respectively. However, in January the first sample site had higher concentrations of all elements except Mn; (107, 22.0, 26.1, 10.3, 1.48, and 0.506 mg/L for Fe, Al, Zn, Cu, Mn, and Pb respectively) when compared to the northern stream (55.0, 10.6, 7.55, 6.10, 1.59, and 0.041 mg/L for Fe, Al, Zn, Cu, Mn, and Pb respectively): but by the second sample site, the southern stream concentrations had dropped to concentrations present in the northern stream. This data indicates less AMD was produced on the southern side during low rainfall periods. Remediation was measured by calculating the percentage reduction in concentration (PRC) between sample sites. PRCs were higher in January for most of the sites; posHow to cite this paper: Marsay, N. (2018) Is the Remediation at Parys Mountain Successfully Reducing Acid Mine Drainage? Journal of Environmental Protection, 9, 540-553. https://doi.org/10.4236/jep.2018.95034 Received: March 16, 2018 Accepted: May 27, 2018 Published: May 30, 2018 Copyright © 2018 by author and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
Britain has a long history of mining for metals, dating back at least 4000 years, this has produced a vast number of mines, with over 3700 sites identified from studies in Wales, the South West and Northumbria alone [1].Many of these sites pose no hazard to the environment; but sadly this is not the case for all sites, with the Water Framework Directive identifying 7% of British water bodies as being potentially at risk [1].With the environment under so much pressure steps need to be taken to identify the most effective methods of remediation.
Once the largest copper mine in Europe, today it lies abandoned, but Parys Mountain remains relevant as it is currently the largest provider of zinc and copper to the Irish Sea, annually discharging of 24 tons of zinc and 10 tons of copper [1].This study aims to identify which of the three environments along the outflows leaving the site are most effective at preventing acid mine drainage (AMD) from entering the surrounding environment.

What Is Acid Mine Drainage?
AMD is the product of water coming into contact with sulfide minerals and being exposed to the atmosphere; it occurs at almost all mines that have sulfide deposits.The same process occurs as a natural process on natural outcrops, this is known as acid rock drainage (ARD).AMD usually occurs in greater quantities than ARD because mining increases the surface area of sulfide minerals exposed to the environment.AMD has many effects on the environment.Firstly, it lowers the pH: at Parys mountain water samples taken from the streams leading into Afon Goch were found to have a pH of 2.8 or lower; this was maintained along a 1 km stretch of the stream [2].This lower pH makes the environment uninhabitable for many flora and fauna but those that can survive the lower pH are threatened by toxic metals, which are more bio-available in low pH environments [3].

How Is Acid Mine Drainage Generated?
The generation of AMD involves multiple reactions and follows different chem-N.Marsay ical routes depending on the pH of the environment.The first step (Equation ( 1)) is the oxidation of pyrite to aqueous iron and sulphuric acid [4]; this reaction lowers the pH.
Based on (Equations ( 1)-( 3)), where the final product is Fe(OH) 3 , it is possible to show the whole process as (Equation ( 5)) [5].This umbrella equation is agreed by most of the scientific community to be an accurate representation of AMD, but it ignores the fact that in the more acidic conditions Fe 3+ is the primary oxidant of pyrite as opposed to oxygen as shown in (Equation ( 4)).(Equation ( 6)) [5] better shows this and also has 8.5 moles of H + per mole of pyrite, as opposed to 4 moles H + per mole of pyrite.
At Parys Mountain Equation (2) will be closer to the truth on site; but downstream of Parys Mountain the acidity reduces, (reaction 3) will be able to occur and hence (Equation (5)) will be more accurate.In reality neither of these umbrella equations show the overall reaction as both ignore part of the process to create a balanced equation [6]; the equations also ignore other minerals involved in the generation of AMD, such as chalcopyrite (CuFeS 2 ) and chalcocite (Cu 2 S) [5], as well as ignoring the increased rates that iron bacteria, such as Thiobacillus Ferro-oxidans and Leptospirillum Ferro-oxidans, can provide, which have both been isolated at Parys Mountain [2].

AMD at Parys Mountain
Despite Parys Mountains historical significance and visible impact on the environment, (Figure 1) there is a limited supply of peer-reviewed research available Journal of Environmental Protection on levels of AMD surrounding the site; the research that does exists was carried out before the controlled breaking of an underground dam in 2003 [1], [7], which altered the flow of AMD leaving the site.
Table 1 shows the mean range of metal concentrations and pH collected by Walton et al.

Hydrology
Parys Mountain has a complicated surface hydrology (Figure 1) with two catchments, one heading north and one heading south, these collect water from the north and south-east sides of Parys Mountain respectively.There are also many settling ponds that appear to have no outflows, presumably draining underground.There is a large network of underground mine shafts [7], of which the details are not publicly available.
During the sampling exercise, the northern stream was fast-flowing, narrow and had no active methods of remediation in place; whereas the southern stream had a slow flow rate; large settling ponds and marshland in its path.The streams, therefore, provide an interesting contrast in remediation potential.

Methodology
To investigate the remediation capabilities of the two main outflows 10 sample sites were selected (Figure 1, Table 2); sites A, B, C, D and E were placed along the southern stream and sites F, G, and H were on the northern stream.All site locations were selected based on ease of access; southern sites were selected to observe the effects of settling ponds and marshes on AMD levels; whereas the northern stream was selected to be at even distances downstream as no interesting features are present on the northern stream.Sampling trips were carried out November water samples were collected in clean Fisher Scientific HDPE 500 mL and January samples were collected in triplicate in 125 mL sample bottles.The sample bottles were flushed twice with the sample then filled completely, sealed and labeled.
The pH of the samples was measured on return to the lab with a Jenway 3510 pH meter.To preserve samples in storage they were then gravity filtered with no.52 Watman paper to remove particulates; followed by the addition of Nitric acid, (HNO 3 70% trace metals basis ≥ 99.999%) 1 drop per 100 ml, to stabilise samples, and stored at room temperature.
The concentration of elements was analysed by Varian 710-ES ICP-OES; samples were diluted 1:9 with deionized water before analysis.
Calibration was carried out by ICP Expert II Software using one method blank and one standard (ICP multi-element standard solution XIII from Merck Millipore) diluted 1:19 with deionized water to provide the concentrations in Table 3.The wavelengths (Table 3) used were selected to avoid interference from elements expected to be present.

Calculation of Percentage Reduction in Concentration
To measure the remediation of elements in the water column, percentage reduction in concentration (PRC) was calculated.This was done by calculating the amount of an element that remained in the lower site as a percentage of the upper site.This value was then subtracted from 100 to give the percentage that had been removed between the first and second site.This is shown in (Equation ( 7)).

pH and ICP
pH values recorded had a range of 2.3 -6.2 (Figure 2).Unlike element concentrations

N. Marsay
Table 3.A list of all elements analysed using ICP-OES, their associated wavelengths used for characterization and concentrations for the standard.For all elements except Lead and Manganese concentrations were higher in the northern stream compared to the southern stream (Figures 3-6).However, these elements in January at site A had a higher concentration than the northern stream but by site B the concentration had decreased to within the range of the northern stream's concentrations (Figures 3-6).
Manganese followed the same trend as most other elements in November however in January both the northern and southern streams had similar values of approximately 1 mg/L (Figure 7).In both months Lead had a substantially     Figure 8.The lead concentrations across north and south streams for November and January.
higher concentration in the southern stream when compared to the northern stream (Figure 8).
When comparing the streams separately across the two months, the northern stream had higher concentrations in November for all elements; and the southern stream had higher concentrations in January for Iron and Copper (Figure 3 and Figure 4) but not Zinc Aluminium Manganese or Lead (Figures 5-8).

Remediation
January's results (Figure 9) had percentage reduction in concentration (PRC) that are similar for each element excluding Manganese but varied across sites.
The southern stream shows higher PRC for all stages except the 2nd settling pond (B to C) when compared to the northern stream.
November's results (Figure 10) were a lot less consistent; Iron and Manganese specifically did not have similar levels to the other metals and in some cases increased in concentration after passing between sites.
Sites A to B and B to C (the settling ponds) showed higher PRC compared to the northern stream some of the time; but only sites C to D and D to E which covered the bog area had constantly high PRC over both months compared to other sites.

Figure 1 .
Figure 1.An Ordnance Survey map highlighting surface water systems and showing sample site locations and appearance [11].
[2]  across a period of 3 years in the 1980's and 90's.His samples focused on the stream that flows from the centre of Parys Mountain through the central ponds to the southern stream over a distance of 900 m.The results showed a decrease in concentration with distance downstream but no other visible trends.Walton's study shows that iron, zinc, lead and arsenic all exceed their environmental quality standards of 0.73, 0.0005, 0.005 and 0.005 mg/L respectively, as set out by the Water Framework Directive[8] [9][10].

StandardFigure 2 .
Figure 2. The pH measurements in November and January which shows the distinct difference between the wetland and other environments.

Figure 3 .
Figure 3.The iron concentrations across north and south streams for November and January.

Figure 4 .
Figure 4.The copper concentrations across north and south streams for November and January.

Figure 5 .
Figure 5.The zinc concentrations across north and south streams for November and January.

Figure 6 .
Figure 6.The aluminium concentrations across north and south streams for November and January.

Figure 7 .
Figure 7.The manganese concentrations across north and south streams for November and January.

Figure 9 .
Figure 9. January's percentage reduction in concentration highlighting the difference in remediation across the different environments at Parys Mountain.

Figure 10 .
Figure 10.November's percentage reduction in concentration highlighting the difference in remediation across the different environments at Parys Mountain.

Table 1 .
on 3 rd of November 2015 and 26 th of January 2016 to investigate variance with rainfall.Average ranges for elements analysed by Walton et al. along a 900 m stretch of streams flowing from the centre of Parys Mountain and converging with the southern stream [2].

Table 2 .
Location of sample sites with distance downstream form highest site on stream, and description of local environment.