Wat. Res. Vol. 25, No. 4, pp. 447-453, 1991 0043-1354/91 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright © 1991 Pergamon Press pie

SEASONAL CHANGES IN THE SANITARY BACTERIAL QUALITY OF WATER DRAINING A SMALL UPLAND

CATCHMENT IN THE YORKSHIRE DALES

COLIN HUNTER 1 and ADRIAN McDONALD 2

ILeeds School of the Environment, The Polytechnic, Leeds, LS2 8BU and 2School of Geography, The University, Leeds, LS2 9JT, England

(First received April 1990; accepted in revised form November 1990)

Abstract--The faecal coliform concentration of overland flow and streamwater within a small catchment in the Yorkshire Dales was monitored over a 2 year period. Strong and consistent seasonal trends in bacterial concentration were found. During the height of summer (mid June to the end of August), faecal coliform concentrations in streamwater were found to be significantly higher than at other times of year. Consistently low concentrations were found during winter months. These trends closely followed those identified for semi-permanent overland flow, suggesting a direct causal link between streamwater quality and the bacterial concentration of overland flow entering the stream channel. The seasonal trends identified were explained with respect to long term changes in the size of the store of enteric bacteria existing in the surface soils of the catchment. Changes in land store size were related to seasonal changes in the frequency and amount of rainfall input to the catchment. The implications of the research findings for the management of upland catchments used for water supply or recreation are discussed.

Key words--uplands, catchments, policy, bacteria, seasonal, overland flow, contributing areas

INTRODUCTION

Despite the now long recognized link between disease outbreaks and the faecal contamination of water supplies (e.g. Craun et al., 1976; McCabe, 1977), relatively little research has focused on the dynamics of sanitary bacteria in upland catchments used for direct public supply or recreation. Such research appears increasingly relevant given the trend for both agricultural and recreational intensification in upland areas of the U.K. in recent years (Littlewood, 1971; Bell and Bunce, 1987), and the acknowledged existence of many old, direct supply reservoirs from which water undergoes only minimal treatment before entering the supply network (IWES, 1981). A small number of individual properties, mainly farms, receive their water supply by direct, untreated abstraction from small upland streams.

The identification and understanding of temporal changes in the bacterial quality of water draining upland catchments is, therefore, of direct relevance to catchment management strategies. Past research has demonstrated that the concentration of enteric indi- cator bacteria in streamwater tends to increase during individual storm events (Kunkle and Meiman, 1967; Matson et al., 1978; McDonald and Kay, 1981). This has been explained either by the enhanced input of bacteria to streamwater from a surrounding land store caused by the generation of storm run-off (Davis et al., 1977), or by the washout of bacteria, existing in stream bed sediments, as stream discharge increases (Kay and McDonald, 1982). The land store

may consist of faecal bacteria existing within or without parent faecal material, and the size of this store will depend upon the balance between faecal input rate, bacterial die-off rate and the rate of removal of bacteria in run-off, at any given time (Jenkins et al., 1984).

Longer term (seasonal) fluctuations in the bacterial quality of streamwater have received almost no attention in the literature. This paper reports the findings of a 2 year study into the faecal coliform concentrations in a small stream in the Yorkshire Dales. Streamwater flow and bacterial concentration data are compared with data gathered through the concurrent monitoring of the faecal coliform concen- tration in overland flows.

MATERIALS AND METHODS

The research site is situated on Pateley Moor in the Yorkshire Dales, about 25 miles north-west of Leeds (Fig. 1). The study site consists of a research plot of 0.5 ha, within a 520ha subcatchment of the R. Skell. The vegetation corresponds to the three components of the general topography: (1) a relatively f la t bracken ( Pteridium aquilinum ) and heather ( Calluna vulgaris ) moor; (2) deeply incised, steep slopes of old, poor oak forest (Ouercus sp.) with bracken understorey; (3) adjacent to the stream, flat and boggy ground dominated by Sphagnum sp. and Juncus sp. A line of springs and seeps at the base of the steep slopes results in the semi-permanent occurrence of overland flow over much of the research plot. The catchment is managed to provide grazing for agriculture (sheep) and sport (grouse shooting). The use of the catchment for informal recreation is common, especially during summer months.

447

448 COLIN HUNTER a n d ADRIAN McDONALD

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Fig. 1. The study area in relation to the North of England and the research sub-catchment.

Streamwater samples were collected from the R. Skell adjacent to the research plot. An Ott automatic stage recorder was installed in the channel at this point to provide continuous streamflow data. Rainfall and other hydro-mete- orological variables were continuously recorded by an auto- matic weather station on the moor top above the research plot. In all, 83 samples were collected from the R. Skell between the autumn of 1984 and the summer of 1986. Sampling frequency varied from 2 to 3 times a week between October and December 1984, to approximately once a week between March and December 1985, and March and August 1986.

Semi-permanent overland flow within the research plot was sampled at the stream bank. This was achieved using both "at a spot" and trough collectors. The former com- prised short, sterilized, 30 cm lengths of rigid plastic tubing inserted approx. 10cm into the top 3cm of bank at lo- cations known to produce frequent overland flow. Nine such sites were inserted along a reach (c. I00 m) of stream bank. Three overland flow collecting troughs were also con- structed within the same stream bank reach. Each trough was constructed by laying a 3 m length of rectangular guttering (10 x 10cm) into a pre-excavated strip of flat ground, such that the collecting edge of the trough was as near as possible flush with the soil surface. A small pit was excavated at the downslope end of each trough to allow the

sampling of collected flow. A total of 61 sets of samples were collected from overland flow sites spanning the same overall time period as previously described for streamwater sampling. Not all overland flow sites could be used at the same time, but in total 438 samples were collected. The sampling of overland flow and streamwater was, as near as possible, concurrent.

All water samples were collected in pre-sterilized, 250 ml, screw-top, pyrex bottles, and analysed in the laboratory for faecal coliform (FC) bacteria. Faecal coliform concentration was determined by membrane filtration using mFC agar plates incubated at 44°C for 24 h (HMSO, 1969). All water samples were processed within 6 h of collection. The authors recognize that other indicator bacteria with different at- tributes could have been employed, but the faecal coliform group is a widely used and accepted indicator, which can be detected quickly and easily.

RESULTS

For each sampling occasion, the arithmetic mean input concentrat ion (AMIC) of faecal coliform bac- teria to the stream channel from overland flow sites was calculated. Thus, changes in the bacterial quality

Seasonal changes in upland bacterial water quality

Table 1. Descriptive statistics of FC concentrations (FC 100 ml-=) in streamwater and overland flow according to seasonal periods (summ.=summer,

aut. = autumn, w/s = winter/spring)

Data Sample used size Minimum Maximum Median Mean

Streamwater summ. 29 4 750 48 162 aut. 25 1 213 9 34 w/s 29 1 18 1 3 all 83 I 750 10 68

summ. 26 11 379 34 87 aut. 17 4 205 22 47 w/s 18 1 43 8 10 all 61 1 379 21 53

Overland flow

449

of overland flow could be compared to those in the R. Skell.

An initial analysis of streamwater and overland flow FC concentration records, revealed consistent seasonal trends. The data were considered in terms of three distinct periods. Although these periods may not conform strictly to any accepted classification of "season", they do highlight the most apparent trends for the presentation of results. The seasonal periods chosen were: summer - -mid June to the end of August; au tumn--beginning of September to mid November; winter /spr ing--mid November to mid June.

Table 1 gives descriptive statistics of FC concen- trations found in streamwater and overland flow. A scatter plot of lOgl0 streamwater FC concentration against stream stage height, with data differentiated according to season, is given in Fig. 2. The logl0 transformation of concentration data follows the convention in the literature (e.g. McDonald and Kay, 1981), and reflects the high degree of variation com- monly observed in field studies of coliform levels. The use of this transformation also increases the normal- ity of the distribution of FC concentration values (Kay, 1979). A normal distribution in the dependent

Log FC conc. ( n o . / 1 0 0 m l )

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20 -3'o 40 6'o Stage height (cm)

• S u m m e r d a t a • Autumn d a t a • W in te r /Spr ing d a t a

Fig. 2. The relationship between log~0 streamwater FC concentration and stream stage height, with data differentiated according to season.

450 COLIN HUNTER a n d ADRIAN McDONALD

variable data set is one assumption of linear re- gression analysis (Poole and O'Farrell, 1971), a tech- nique which is utilized later in this paper.

It is clear from Table 1 and Fig. 2 that streamwater FC concentration tended to be highest during the summer and lowest during winter/spring, with inter- mediate values found during the autumn. This pat- tern was also found for FC concentrations in overland flow. Given the sensitivity of the arithmetic mean to extreme observations, the median values in Table 1 provide more reliable indicators of central tendency. For streamwater FC concentration, an overall median concentration of 10 FC 100 ml ~ was found. During the summer, median concentration was 48 FC 100ml- ' , while the corresponding win- ter/spring value was only 1 FC 100 ml-1. The overall median FC concentration in overland flow was found to be 21 FC 100 ml-~, while median values during the summer and winter/spring periods were 34 and 8 FC 100 ml - ~, respectively.

Mann-Whitney U-tests were carried out for streamwater FC concentration between seasonal data set pairings. The null hypothesis in each case was that there was no statistically significant difference be- tween the median values of the populations from which the samples were drawn. For each seasonal data set pairing, the attained significance level (P) was found to be less than 0.0001. Therefore, the null hypothesis could be rejected. No significant differ- ences were found between years for the same seasonal period.

Figure 2 shows that high summer concentrations occurred even under stream baseflow (i.e. non-storm) conditions. This is a finding not previously reported. A strong, positive relationship between FC concen- tration and stream stage height might reasonably have been expected, based on previous reports in the literature (e.g. Matson et al., 1978; McDonald and Kay, 1981) which concentrated on storm event sampling. Least squares linear regression analysis conducted between lOgl0 FC concentration and stream stage height using all stream data, revealed a product-moment correlation coefficient (r) of only 0.035, which is not significant.

The scatter plot in Fig. 3, shows the relationship found between log j0 streamwater FC concentration and lOgl0 AMIC. It is clear that a strong, positive association existed between the bacterial concen- tration in overland flow entering the channel, and the bacterial quality of streamwater. The linear regression equation corresponding to Fig. 3, revealed a correlation coefficient of 0.781 (significant at P < 0.0001), a gradient close to unity (1.03), and a y-axis intercept close to zero (-0.11) .

The technique of multiple linear regression analysis was employed as a method of further analysing the factors involved in explaining the observed changes in the FC concentration of overland flow and stream- water. The aim was to determine if a set of environ- mental parameters could be formulated which adequately described these changes in water quality. Logically, current and recent rainfall and streamflow

Log stream conc. (noJ 100ml) 3 .20 -

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I • • / I I 0- - 0.80 1.60 2.40 3.20

Log A M I C (no./lOOml)

Fig. 3. The relationship between ]og~0 strcamwater FC concentration and ]ogt0 AM]C in overland flow.

Seasonal changes in upland bacterial water quality

conditions will be important determinants of changes in the FC concentration in overland flow and, there- fore, streamwater (Kay, 1979; Jenkins, 1984). Par- ameters, such as the amount and timing of rainfall events prior to sampling, which reflect these con- ditions can reasonably be expected to explain a large proport ion of the variance in log~0 A M I C and lOgl0 streamwater FC concentration.

The independent variables chosen for the multiple regression analyses are given in Table 2. The rationale for the inclusion of most of the variables in Table 2 is self explanatory, being designed to reflect long and short term changes in bacterial loss from the catch- ment land store. The choice of the absolute values in the variables T R F I - T R F 4 are arbitrary, but these values were seen as corresponding to degrees of recent rainfall input and, therefore, bacterial loss from the land store. The same applies to variable TST, with the value of 23 cm chosen to represent baseflow stream conditions. Air temperature data was included (variable TMP) because this may be taken as an indicator of 10ng term survival conditions for faecal bacteria in the land store (Gerba et al., 1975). The relative stocking density variable (RSD) was set at a value of 2.5 between May and August, and at a value of 1.0 at all other times. This was designed to reflect seasonal differences in faecal bac- terial input from sheep, as a result of lambing (Bell and Bunce, 1987).

A correlation coefficient matrix was calculated using all independent variables. This was carried out to reveal the extent of any potential problem associ- ated with multi-collinearity between predictor vari- ables. The highest correlation coefficient found was 0.796, between variables ST and RF2. This value is lower than r = 0.800, the value suggested by Kim and Kohout (1975) as representing extreme multi- collinearity. No independent variable in Table 2 was excluded from the multiple regression analysis pro- cedure, on the basis of multi-collinearity.

With reference to other assumptions made when using the multiple linear regression technique (see, for example, Poole and O'Farrell , 1971), the authors recognize that these requirements are not met in full. Clearly, a variable which is composed of a set of environmental observations will not comply com-

Table 2. The independent variables utilized in the multiple regression analyses

Variable name Description

RF1 RF2 RF3 TRF1 TRF2 TRF3 TRF4 ST "ST

Rainfall total for the 4 h prior to sampling (mm) Rainfall total for the day prior to sampling (mm) Rainfall total for the week prior to sampling (mm) Time since daily rainfall > I mm (days) Time since daily rainfall >3 mm (days) Time since daily rainfall > 10 mm (days) Number of days in last 10 when rainfall > 1 mm Stream stage height at the time of sampling (cm) Time since stream stage height > 23 cm (days) Relative sheep stocking density Mean air temperature for the day prior to sampling CC)

451

Table 3. The results of multiple regression analysis for logL0 AMIC (FC 100 ml -~ ) in overland flow

Coefficient Multiple R 2 Variable (B) SE B R 2 change

RF1 0.1036 0.0203 0.248 0.248 TST 0.0292 0.0068 0.415 0.167 RF2 0.0244 0.0049 0.570 0.155 TMP 0.0357 0.0174 0.594 0.024

(Const. = 0.6716). F ratio = 26.287, significant at P = <0.001.

pletely with all the assumptions associated with the technique. A lack of complete compliance with these assumptions does not mean that the use of the technique is invalid. The technique is accepted as being relatively " robus t" (e.g. Kay, 1979), and was used in this study for investigative rather than predic- tive purposes.

Multiple regression equations were determined for both log~0 A M I C and log~0 streamwater FC concen- tration. For the determination of the log~0 stream- water FC concentration equation, only the data for which there were corresponding overland flow results were used. Thus, 61 observations were used for the determination of both the log10 A M I C and log~0 streamwater FC concentration equations. No at- tempt was made to use seasonal groupings of data for this part of the analysis. Independent variables were entered into the multiple regression analysis pro- cedure according to the degree of correlation between it and the dependent variable, with high correlation independent variables entered first. In the production of the final regression equation, an independent variable which accounted for less than 2% of the total variance explained (i.e. the multiple R 2 value) was not included.

The results of multiple regression analysis for log~0 A M I C and log~0 streamwater FC concentration are shown in Tables 3 and 4, respectively. Both regression equations were found to be highly significant (P < 0.001), and included the same three variables (RF1, RF2 and TST). The R F I and RF2 variables clearly reflect current and recent rainfall conditions, while the time variable, TST, was apparently very important in explaining high summer concentrations which often occurred under baseflow conditions (see Fig. 2). The equation for lOgl0 A M I C also included the temperature variable, T M P (see Table 3), giving a total variance explained (multiple R 2) value of 59.4%. The fourth variable included in the equation for log~0 streamwater FC concentration was relative

Table 4. The results of multiple regression analysis for Iog~0 streamwater FC concentration (FC 100 ml ~)

Coefficient Multiple R 2 Variable (B) SE B R 2 change

TST 0.0413 0.0079 0.229 0.229 RF1 0.1118 0.2046 0.403 0.174 RSD 0.2761 0.0941 0.485 0.082 RF2 0.0336 0.0060 0.639 0.154

(Const. = 0.2937). F ratio = 31.921, significant at P = <0.001.

452 COLIN HUNTER and ADRIAN MCDONALD

stocking density, RSD (Table 4), and the total vari- ance explained was 63.9%.

DISCUSSION

It is interesting and useful to compare the FC concentration record of the R. Skell with bacterial water quality standards. The European Community has produced widely applicable, epidemiologically based guideline standards for waters used for public supply (EEC, 1975) or recreation (EEC, 1976). The guideline value for waters receiving minimal treat- ment before entering the domestic supply network is 20 FC 100ml -~. For recreational waters, the guide- line value is 100 FC 100ml -l . These guidelines are based on a fortnightly sampling frequency. Using all the streamwater FC concentration data, the former guideline value was exceeded in 35% of samples, while the latter was exceeded in 17% of samples. The relatively high contravention frequency of the guide- lines is surprising given the non-intensive land use which prevails in the catchment, and does not augur well for the more intensive recreational or agricul- tural use of upland drainage basins. Furthermore, it was in fact rare for samples collected from mid June to August to contain fewer than 20 FC 100 m1-1, as indicated by the median concentration of 48 FC 100ml J for the summer season given in Table 1. Unfortunately, the summer season is the period of maximum recreational activity, and thus the period of maximum potential risk of disease transmission. This is also the season when reservoirs are most depleted through drawdown, have the lowest resi- dence time for influent waters, and the potential for the dilution of streamwater and diffuse catchment run-offis least (Kay, 1979; Jenkins, 1984). The risk of supplying water of unsatisfactory quality would, therefore, appear to be greatest during the summer.

The seasonal trends identified for streamwater FC concentration closely mirrored those found for the FC concentration of overland flow. This provides the best evidence to date that the dynamics of sanitary bacteria in the catchment land store (as reflected by changing concentrations in overland flow) exert a direct influence on bacterial levels in streamwater. An explanation of observed trends in bacterial water quality may be found in seasonal changes in rainfall characteristics over the catchment, as suggested from the results of multiple regression analysis. A full description of seasonal changes in rainfall character- istics may be found in Chandler and Gregory (1976). With the onset of winter, and the larger and more frequent inputs of rainfall associated with this season, it appears reasonable to conclude that a slow and progressive depletion of the bacterial land store oc- curred. Thus, FC concentrations in overland flow and streamwater between late November and early June were found to be relatively low. Between mid June and August, however, rainfall input was lower and less frequent, while evapotranspiration was at a maxi-

mum, thus allowing substantial recovery in land store size. (The degree of recovery being indicated by the variable TST utilized in multiple regression analysis.) This recovery combined with the existence of an effective bacterial transport mechanism in the form of semi-permanent overland flow, resulted in relatively high streamwater FC concentrations even under baseflow conditions.

The results of this research suggest that upland streamwater bacterial quality may be poor at certain times of year. The most obvious potential solution to this problem would appear to be to reduce substan- tially the influence of semi-permanent overland flow. This could be achieved by lowering the water table close to stream channels through the installation of field drainage systems. The long term effect of such action should be to reduce the spatial occurrence and quantity of overland flow, and reduce bacterial input at the channel as a higher proportion of total water input is subject to the filtering action of the soil. An ecologically more acceptable alternative, which avoids habitat destruction, might be to exclude sheep or cattle access (by fencing) from those catchment areas most frequently involved in the generation of overland flow. This concept of the identification of "bacteria sensitive areas" parallels the identification of acid sensitive areas (e.g. in Wales) and nitrate sensitive areas. Each form of sensitive area requires a particular management strategy to control the specific problem.

Acknowledgements--The authors are indebted to Bostock Estates for permission to work on their land. We thank Mandy Kelly for her work in both field and laboratory, and Tim Hadwin and John Dixon for the preparation of dia- grams. Our thanks also go to the Institute of Hydrology for providing the Automatic Weather Station. The research was funded by an NERC studentship.

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