3.4 Concentrations of elements in rainwater

The annual precipitation-weighted mean concentrations of the 25 trace elements, nitrate and sulphate measured at each of the three sites during the years 1996, 1997 and 1998 are listed in Table 8. For comparison, the corresponding data for the previous two years are also presented in Table 8. The concentrations of many elements were below analytical limits of detection during 1992 and 1993 (Baker, 1997) and have therefore been excluded from this comparison. However, improvements in analytical techniques since 1993 have enabled positive results to be reported for many more of the metals; in particular, As, Cd and Co.

The elements can be broadly classified, according to rainwater concentrations (soluble fraction only) that were usually measured:

(i) Concentrations >1000 m g l-1: Na (also nitrate and sulphate).

(ii) Concentrations between 100 - 1000 m g l-1: Ca, K, Mg.

(iii) Concentrations between 10 - 100 m g l-1: Al, Zn.

(iv) Concentrations between 1 - 10 m g l-1: Cu, Fe, Mn, Pb.

(v) Concentrations of <1 m g l-1: As, Cd, Co, Cr, Mo, Se, V, W.

Rainwater concentrations of Ni, Sb and Ti fell into groups (iv) or (v) depending on location (see Table 8). The rainwater concentrations of the remaining four elements that are routinely measured were frequently below analytical limits of detection (see Table 8).

The annual deposition of some elements, together with nitrate and sulphate has been estimated for each of the years during 1994-1998 (Table 9). The estimates were based on rainwater concentrations and measurements of rainfall. Of all the elements that are routinely measured, only those elements whose annual mean concentrations were regularly above analytical detection limits have been included in this summary.

The quarterly mean rainwater concentrations of certain elements for the period 1994-1998 have also been plotted. The heavy metals As, Cd, Co, Cu, Pb and Zn at Chilton, Styrrup and Wraymires are shown in Figure 11a. Ni and V were omitted since some quarterly concentrations were below prevailing analytical limits of detection, particularly during 1995 and 1996. Figure 11b shows the corresponding concentrations of Al, Ca, Fe, K, Mg, Mn and Na.

The high annual rainfall at Wraymires, about three times greater than at both Chilton and Styrrup, should be noted when making comparisons between concentrations of elements in rainwater at the three sites. Also, it should be remembered that any dry deposition to the collecting funnel will be washed into the rain bottle.
 
 

Table 8 Annual Precipitation-Weighted Mean Concentrations of Elements, Nitrate and Sulphate in Rainwater at Chilton, Styrrup and Wraymires (1994-1998)

Table 9 Summary of Estimated Annual Deposition of Elements, Nitrate and Sulphate at Chilton, Styrrup and Wraymires (1994-1998)

Figure 11a Quarterly mean concentrations of heavy metals in rainwater at Chilton, Styrrup and Wraymires (1994-1998)

Figure 11b Quarterly mean concentrations of Al, Ca, Fe, K, Mg, Mn and Na in rainwater at Chilton, Styrrup and Wraymires (1994-1998)

Concentrations of heavy metals in rainwater at rural locations in the UK

Concentrations of several heavy metals in rainwater are routinely measured at the three east coast rural locations (East Ruston, High Muffles and Banchory). The average, maximum and minimum precipitation-weighted annual mean concentrations in rainwater, measured at these sites for the period 1994-1998, are compared with those from Chilton, Styrrup and Wraymires in Table 10.

These concentrations (Playford and Baker, 1999) are for bulk deposition (and include the insoluble rainwater fraction). Historical measurements (1972-1981) made at Chilton, Styrrup and Wraymires showed that most of these heavy metals are associated with the soluble fraction of rainwater (Cawse et al., 1995).
   

Table 10 Annual Precipitation-Weighted Mean Concentrations of Heavy Metals in Rainwater at Rural Locations in the UK (1994-1998)

Location  
Annual
Mean
Rainfall
(mm)
Annual Mean Rainwater Concentration (m g l-1)
     
As
Cd
Cu
Cr
(b)
Ni
Pb
Zn
Chilton Mean
683
0.26
0.16
6.7
0.054
0.91
1.8
21
  Max
805
0.41
0.30
10.8
0.080
1.1
2.8
47
  Min
468
0.11
0.064
2.4
0.040
0.31
0.26
6.5
Styrrup Mean
578
0.50
0.32
8.7
0.26
1.8
4.7
43
  Max
705
0.78
0.61
14.1
0.43
3.8
7.7
79
  Min
474
0.22
0.15
2.1
0.084
0.42
1.5
21
Wraymires Mean
1927
0.14
0.14
7.5
0.11
 
1.8
10
  Max
2453
0.23
0.25
11.6
0.24
(c)
2.6
18
  Min
1537
0.058
<0.03
2.6
0.014
 
0.67
6.0
East Ruston (a) Mean
675
0.23
0.11
2.2
0.40
1.0
3.9
18
  Max
764
0.29
0.20
3.4
0.52
1.7
5.4
30
  Min
598
0.15
0.06
1.2
0.24
0.50
2.2
9.3
High Muffles (a) Mean
788
0.32
0.09
1.6
0.34
0.62
4.0
20
  Max
997
0.42
0.13
2.1
0.57
0.97
5.3
29
  Min
617
0.20
0.05
1.0
0.16
0.18
2.6
10
Banchory (a) Mean
877
0.20
0.11
1.0
0.44
0.41
2.1
12
  Max
1040
0.31
0.25
1.2
0.63
0.66
3.1
23
  Min
680
0.09
0.04
0.85
0.23
0.16
0.90
5.2

Notes

(a) Measurements exclude 1996.

(b) In 1995 and 1996, the annual mean concentrations of Cr at Chilton, Styrrup and Wraymires were below analytical limits of detection (<0.8 mg l-1). These have been excluded in the above summary.

(c) The majority of the annual mean concentrations of Ni at Wraymires were below analytical limits of detection (typically <0.1 mg l-1). Therefore, these data have been excluded from the above summary.
 
  Over the last five years, the average annual mean rainwater concentrations of As, Cd, Pb and Zn at Chilton and Wraymires were similar to those at the three east coast locations. Average concentrations of these four metals were slighted elevated at Styrrup in comparison (Table 10).

Average Ni concentrations at Chilton and Styrrup were also of a similar order to those measured at the east coast sites, with slightly elevated levels found at Styrrup. At Wraymires, annual mean concentrations of Ni were mainly below analytical detection limits (i.e. <0.1 m g l-1).

For Cr, due to the presence of some data (from Chilton, Styrrup and Wraymires for 1995 and 1996) that were below the prevailing analytical detection limits (typically <0.8 m g l-1), a comparison with the other three locations was more limited. The highest concentrations of Cr measured at Chilton, Styrrup and Wraymires were of the same order as those recorded at Banchory, East Ruston and High Muffles. However, the lowest were an order of magnitude lower at the inland sites (Table 10).

The concentrations of Cu in rainwater measured at Chilton, Styrrup and Wraymires during the period 1994-1998 have been consistently similar at all three of these locations (Table 10). The average concentrations at these sites were typically 3 to 4-fold higher than those measured at East Ruston and Banchory and up to 8 times greater than those at Banchory (Table 10).

Elevated concentrations (2 to 3 times greater) were measured at Chilton, Styrrup and Wraymires relative to the three coastal sites during the period 1992-1996. Also, the annual mean concentrations for the period 1987-1991 at Chilton, Styrrup and Wraymires were consistent with those for 1992-1996 (Baker, 1997). Therefore, the increased Cu concentrations in rainwater at Chilton, Styrrup and Wraymires relative to the other three rural sites is not easily explained.
   

Concentrations of nitrate and sulphate in rainwater

The annual mean concentrations of nitrate and sulphate measured at Chilton, Styrrup and Wraymires during 1994-1998 were of the same order as those recorded at nearby sites from the Acid Deposition Monitoring network (Campbell et al., 1998; Vincent, 1999). (Table 11). These sites are at Compton (Berkshire), Thorganby (Yorkshire) and Bannisdale (Cumbria), respectively. The sites are separated by distances of approximately 7 km, 40 km and 18 km, respectively.

Although the concentrations of both nitrate and sulphate at Chilton, Styrrup and Wraymires were of a similar magnitude to the corresponding Acid Deposition network sites, the higher concentrations tended to be recorded at the trace element network sites (Table 11). The greatest difference was found between nitrate concentrations at Chilton and Compton, where average concentrations were 1.8 times greater at the former. Average concentrations were more comparable at the Wraymires and Bannisdale sites, with only 20% differences in the annual means for the period 1994-1998 apparent.

These differences are likely to arise from operational differences between the two networks, e.g. in sampling frequency and analytical procedures. Sampling of nitrate and sulphate ions are not of primary interest in the Rural Trace Elements network which was mainly established for the measurement of airborne trace and major elements. Nevertheless, the measurement of sulphate and nitrate ions provides a useful additional dataset to the Acid Deposition Monitoring networks.
   

Table 11 Annual Precipitation-Weighted Mean Concentrations of Nitrate and Sulphate in Rainwater at Rural Locations in the UK (1994-1998)

Location
Annual Mean Rainfall (mm)
Annual Mean Rainwater Concentration (m g l-1)
   
NO3-
SO42-
Chilton
683
3958
3575
Styrrup
578
3430
4432
Wraymires
1927
1732
1862
Compton
573
2152
2893
Thorganby
427
2336
3476
Bannisdale
1864
1490
2500