Environmental Monitoring and Assessment (2005) 111: 297Ð306 DOI: 10.1007/s10661-005-8224-3 c Springer 2005

THE WATER QUALITY OF THE CERTIMA« RIVER BASIN (CENTRAL )

M. A. CERQUEIRA∗,F.N.VIEIRA, R. V. FERREIRA and J. F. SILVA Departamento de Ambiente e Ordenamento, Universidade de Aveiro, Aveiro, Portugal (∗author for correspondence, e-mail: [email protected])

(Received 15 April 2004; accepted 23 December 2004)

Abstract. The aim of this study was to evaluate the water quality of the C«ertima River basin (Central Portugal). For that purpose, surface water samples were collected in March, May and July 2003, at 10 selected sampling sites, and were analysed for physicochemical parameters, namely temperature, conductivity, pH, total suspended solids, dissolved oxygen, biochemical oxygen demand (BOD5), Kjeldahl nitrogen and total phosphorus. Results revealed an acceptable water quality during the −3 −3 spring season. Maxima of 64 mg dm for BOD5,39mgdm for Kjeldahl nitrogen, and 5.2 mg dm−3 for total phosphorus, were recorded during summer, indicating a significant degradation of the water quality in a river stretch located downstream of the town of Mealhada. These values, which did not comply with the objectives of minimum quality for surface waters prescribed by the Portuguese legislation, were related to domestic wastewater discharges and runoff waters from a cattle farm. Besides their effects on the middle stretch of the river, these pollution sources were the most likely cause of the high nutrient load in downstream waters, and thus may have a major impact on the trophic status of Pateira de Fermentelos, a sensitive wetland area located in the lower C«ertima basin.

Keywords: environmental monitoring, pollution, Portugal, river, water quality

1. Introduction

Freshwater resources are abundant in Portugal, about 72 km3 (FAO, 2004), com- prising 92% of surface waters and 8% of ground waters. However, these resources are unevenly distributed in the territory, since precipitation is more frequent in the northern part of Portugal, and also strongly dependent on the seasonal variation of climate, given that rainfall is mostly concentrated in the winter period. The agricul- tural sector is the most important water consumer with about 87% of the total fresh- water withdrawals, while industry and domestic supply are responsible for 8 and 5%, respectively, of the overall water consumption (IA (Instituto da Agua),« 2001). Portuguese water resources have been under a continuous threat of pollution because of the absence or insufficient levels of sewage treatment (Borrego, 1993), diffuse contamination from agriculture (Cerejeira et al., 2003) and mining activities (Monteiro, 1995). Although water pollution resulting from industrial activities and domestic use has decreased during the last decade, thanks to large investments in sewage treatment plants, much more efforts are still needed to completely control pollution focuses. Recent statistical data indicate that sewage drainage systems 298 M. A. CERQUEIRA ET AL. and sewage treatment plants served only 64 and 37%, respectively, of the resident population in the year 2000 (IA, 2001). In this context, water-quality monitoring is a helpful tool not only to evaluate the impacts of pollution sources but also to ensure an efficient management of water resources and the protection of aquatic life. Portugal has a water-quality monitoring network that covers the whole country, but sampling stations are predominantly located in the most important watercourses and, for that reason, information about smaller drainage basins is still scarce. Ac- cordingly, the aim of the present study was to perform a detailed seasonal survey of the water quality of the C«ertima River basin, a small system that drains into the Vouga River, the third longest watercourse flowing exclusively within Portugal. This task was accomplished by monitoring of physicochemical variables in surface water samples. Results were then compared with the objectives of minimum quality defined on the Portuguese legislation (Decreto-Lei no 236/98). This study is also relevant considering that the C«ertima River is the main source of water to the lake of Pateira de Fermentelos, a wetland classified as a sensitive area by the Decreto-Lei no 153/97, which transposes to the Portuguese law the European Directive 91/271/CEE concerning urban wastewater treatment.

2. Experimental

2.1. SAMPLING AREA

The C«ertima River is a sub-tributary of the Vouga River, a watercourse that drains into the Atlantic Ocean via the coastal lagoon of Ria de Aveiro (Figure 1). It arises on the western slope of Bussaco Mountain, at an altitude of 380 m, and flows in a dominant northern direction for about 43 km, crossing the municipalities of Mealhada, Anadia, and Agueda.« In the lower reaches, the valley of the river opens widely to form the shallow lake of Pateira de Fermentelos, narrowing again near the village of Requeixo, where the C«ertima discharges into the Agueda.« The C«ertima catchment covers a total area of approximately 510 km2, and its underlying geology consists of Ordovician age schist in the uplands (to the east), and Modern age alluvial sands and clays in the middle and lowlands (to the west). The hydrologic regime of the C«ertima River reflects the annual variation of cli- mate, which is of the Mediterranean type with a strong influence from the Atlantic Ocean. Summers are dry and hot, due to the effect of the Azores high-pressure system. Winters are mild and wet, with strong precipitation events, resulting from the passage of frontal surfaces and depressions with Atlantic origin. The average discharge into the River Agueda,« estimated from the difference between precipi- tation and evapotranspiration, is 6 m3 s−1. The effect of summer climate on the hydrologic regime is so severe that, in a typical year, the riverbed dries up for a WATER QUALITY OF THE CERTIMA« RIVER BASIN 299

Figure 1. Location of the sampling sites within the C«ertima River basin (Central Portugal). length of about 20 km, from the source to the vicinity of Mogofores (site 5 in Figure 1). The slope of the C«ertima channel decreases from values larger than 5 × 10−3, upstream of Mealhada, to values smaller than 5 × 10−4 in the lower reaches, near the lake of Pateira de Fermentelos. However, the middle stretch of the river presents afairly constant slope, ranging between 2 × 10−3 and 3×10−3.Inthis stretch flow rate increases moderately because of the contribution of three tributaries draining the east side of the basin. The channel has a uniform cross-section, delimited by steep banks, and the bottom sediment is mainly fine sand, colonized by aquatic plants at some points. These features imply a moderate roughness, which adds to the above-mentioned slope in promoting low flow conditions. Land cover ranges from forests of Pinus pinaster and Eucalyptus globulus in the upper basin to agricultural fields and pastures in the lower basin. The main industries are from the ceramics, wine production and metal-processing sec- tors. The total resident population within the catchment limits is about 85 000, with an average population density of 160 inhabitants per km2 (INE (Instituto Nacional de Estat«õstica), 2002). Drainage systems are restricted to the major ur- ban settlements, serving 52% of the resident population in the basin, and only a small part of the collected wastewater receives treatment before discharge in the watercourses. 300 M. A. CERQUEIRA ET AL.

2.2. SAMPLING SITES

The location of the 10 selected sampling sites in the C«ertima River basin is shown in Figure 1. The sites were chosen mainly due to their proximity to suspected pollution sources and due to their ease of access. Site 1 (P«ovoa do Loureiro) was the closest site to the source and was selected to represent the reference condition of the water, without interference from human activities. Site 2 (Viadores) was located downstream of the town of Pampilhosa and of the small industrial area of Viadores. Site 3 (Lagoa Seca) was situated after the confluence of the Luso River with the C«ertima River and downstream of the points where the partially treated sewage from the town of Mealhada and the wastewaters from a cattle farm were discharged. Site 4 (Alf«eloas) was located on the Serra River, near the urban settlement of Anadia. Site 5 (Mogofores) was established downstream of the confluence of the Serra River, to monitor the effect of this tributary on the water quality of the C«ertima River. Site 6 (S˜ao Jo˜ao da Azenha) was located in a rural area and was chosen to observe possible effects from the surrounding farming activities. Site 7 (Landiosa) wasonthe Cadaval Stream, just before its conjunction with the C«ertima River. The lower stream drains a moderately populated area with a few industries. Site 8 (Repol˜ao) was intended to monitor possible impacts of domestic and industrial wastewater discharges from the town of Oliveira do Bairro. Site 9 (Alagoa) was located on the lower stretch of Levira River, downstream of Vila Verde, a village with a significant activity of ceramics industries. Site 10 (Perr˜aes) was chosen to observe the effect of flux from the Levira tributary, and to evaluate the quality of the water discharged into the lake of Pateira de Fermentelos. All the samples were collected from bridges, except on site 7, where sampling was performed from the left bank of the Cadaval Stream.

2.3. SAMPLING AND ANALYTICAL METHODS

Field work took place on 26 March, 14 May and 23 July 2003, covering distinct hydrological conditions. Samples from the first 20 cm of the water column were col- lected at each site using a bottom-weighted polyethylene flask, previously washed with water from that site. Temperature and dissolved oxygen were measured in the field with a portable meter equipped with a temperature sensor and a membrane electrode. The samples were then transferred to 2.5 dm3 acid-washed polyethy- lene bottles, transported to the laboratory and processed within 6 h after collection for the analysis of pH, conductivity, total suspended solids (TSS) and biochemi- cal oxygen demand (BOD5). Aliquots of the initial samples were also preserved by the addition of acids for the later analysis of Kjeldahl nitrogen (Kjeldahl-N) and total phosphorus. Conductivity and pH were determined by electrometry. Total suspended solids were assayed by gravimetry after filtration of an adequate sample ◦ volume through a glass fibre filter and drying at 105 C. BOD5 was determined as the difference between initial and 5-day dissolved oxygen content, after incubation WATER QUALITY OF THE CERTIMA« RIVER BASIN 301 of samples in the dark at 20 ◦C. Kjeldhal-N was measured by digestion and dis- tillation of samples, according to the Kjeldhal method, and subsequent analysis of ammonium by the colorimetric indophenol blue method. Total phosphorus was measured as orthophosphate by the colorimetric ascorbic acid method after acid digestion of samples. All the analytical protocols used during this study were in accordance with the standard methods described in Eaton et al. (1995).

3. Results and Discussion

The spatial and temporal variations of the parameters monitored in the surface waters of the C«ertima River basin are shown in Figures 2 and 3. Temperature values were in the range 12.8 ◦C and 14.3 ◦CinMarch, 16.2 ◦C and 19.8 ◦CinMay, and 19.1 ◦C and 22.9 ◦CinJuly, with the minima, for the three campaigns, observed near the source and the maxima in the lower stretch of the river. These were expected records as they reflect the changes in the altitude of the sampling sites and the seasonal variation of climate. Conductivity was low in samples collected from the initial stretch of the C«ertima River and from tributaries of the right bank (sites 1, 4 and 7) because these water- courses drain areas with low population densities, where pollution sources are of scarce relevance. For the sampling sites 2, 3, 4, 5 and 6, conductivity increased by a factor of 2 from March to July. This may be explained, at least in part, with higher evaporation and lower precipitation values in the drainage basin during the warmer season. The maximum conductivity value was recorded in summer, at sampling site 3, just downstream of the town of Mealhada, suggesting a significant impact from wastewater discharges on the river water quality. pH results did not show significant spatial or seasonal differences. The observed values varied between 6.4 and 8.2, falling within the legal limits presented in the Portuguese legislation (5.0Ð9.0). The measurements performed in the water samples collected from the river basin revealed levels of TSS less than 30 mg dm−3 for sites 1Ð8. As expected, the lowest values were recorded at the headwater site. On the other hand, the highest values were recorded in the Levira River (site 9) and downstream of this tributary (site 10). This feature was probably due to wastewater discharges from the ceramics industries located in the area of Vila Verde (upstream of site 9) and may also result from the re- suspension of material accumulated in the lower reaches of the C«ertima river basin. Aregulatory level was not defined in the Portuguese law for TSS in surface waters. As can be seen from Figure 3, the surface waters of the basin were generally well oxygenated. The only exceptions to this were the sampling sites 2 and 3, in summer, with percentages of saturation lower than the objectives of minimum qual- ity for surface waters prescribed by the Portuguese law (Decreto-Lei no 236/98), which is 50%. The result from site 2 can be attributed to sewage discharges from the town of Pampilhosa and to a prominent decrease in the flow rate during the warmer 302 M. A. CERQUEIRA ET AL.

Figure 2. Spatial and temporal variations of temperature, conductivity, pH and total suspended solids in the surface waters of C«ertima River basin. Information for site 1 in July is not available because the riverbed was dry. WATER QUALITY OF THE CERTIMA« RIVER BASIN 303

Figure 3. Spatial and temporal variations of dissolved oxygen, BOD5, Kjeldahl-N and total phospho- rus in the surface waters of C«ertima River basin. Information for site 1 in July is not available because the riverbed was dry. 304 M. A. CERQUEIRA ET AL. season. In this area the course of the river was transformed into a succession of small ponds, with almost still water, thus limiting the oxygen transfer by aeration. In turn, the oxygen depletion observed at site 3 clearly reveals the occurrence of an elevated content of organic matter in the waters, resulting from the combined effect of a low flow rate with non treated wastewater discharges coming from the urban area of Mealhada and from a cattle farm located southwest of the sampling site. Also interesting to note in Figure 3 are the results from sites 5 and 6, with values higher than 100%. Release of oxygen into the waters resulting from the photosyn- thetic activity of plants can explain the observed levels, as aquatic vegetation was particularly abundant in a segment of the river that comprised those sites. These observations illustrate the limitations of using dissolved oxygen as an indicator of organic pollution in small rivers like the C«ertima. Measurements of BOD5 revealed a significant contamination by organic matter in a segment of the C«ertima River comprised between Pampilhosa and Mogofores (sites 2, 3 and 5), in clear agreement with the depletion of dissolved oxygen in the waters. This condition was very critical near the town of Mealhada, during −3 summer, where the BOD5 levels exhibited a peak of 64 mg dm Ð well above the legal limit of 5 mg dm−3 Ð pointing the outstanding effect of the aforementioned pollution sources. A considerable improvement on the water quality was observed −3 going downstream from site 3 to 5 (even though BOD5 was higher than 8 mg dm during the dry period). This can be attributed to a very fast self-purification of the waters and, in a minor extent, to the dilution effect that resulted from the confluence −3 of the Serra River (summer BOD5 <2mgdm ) with the C«ertima River. Water −3 pollution with organic matter (BOD5 levels ranging between 3 and 6 mg dm )was also reported to occur in other small rivers draining into the north side of the Ria de Aveiro coastal lagoon, a consequence of limited sewage collection and treatment systems (Silva et al., 2002). The N-Kjeldahl concentrations recorded at sites 3 and 5 on July were 39 and 4.6 mg dm−3, respectively. These were the highest values of the whole set of results gathered throughout this study and were far in excess of the Portuguese standard of 2mgdm−3. High levels of N-Kjeldahl are indicative of a poor sanitary condition of the waters due to a fresh contamination with organic wastes (Hooda et al., 2000; Sawyer et al., 1994). Therefore, these results clearly sustain the occurrence of wastewater discharges upstream of site 3. The distribution of total phosphorus in the waters was quite similar to that of BOD5 and N-Kjeldahl. Once again the peak value was recorded downstream of Mealhada, with a summer concentration about five times higher than the maximum admitted value presented on the Decreto-Lei no 236/98, which is 1 mg dm−3. This indicates that wastewater discharges from the town of Mealhada were the most likely source of phosphorus in the waters, as this species is an important constituent of domestic detergents and is not released in significant amounts by animals. Most of the samples collected from the lower river basin (sites 8, 9 and 10) revealed N-Kjeldahl and total phosphorus concentrations around 0.1 mg dm−3 and WATER QUALITY OF THE CERTIMA« RIVER BASIN 305

0.7 mg dm−3 respectively, which are indicative of a moderately polluted condition of the surface waters (Drolc and Zagorc Koncan, 2002; Fytianos et al., 2002; Silva et al., 2002). The enrichment of nutrients in these samples clearly suggests that the C«ertima River can affect significantly the primary productivity and trophic status of Pateira de Fermentelos. However, the input of nitrogen and phosphorus into this lake is only a small fraction of the total load discharged through wastewaters, indicating that a significant amount of nutrients is consumed in the middle stretch of the river.

4. Conclusions

The present study revealed a significant degradation of the water quality in a stretch of the C«ertima River located downstream of the town of Mealhada, where summer values for BOD5, N-Kjeldahl and total phosphorus did not comply with the objec- tives of minimum quality for surface waters set by the Portuguese legislation. This state resulted from the combination of summer low flow river conditions with dis- charges of untreated domestic sewage and runoff waters from a cattle farm. During that period the extent of organic pollution was so serious that dissolved oxygen became almost absent from the waters. The impact of high amounts of nutrient discharges into the river was evident from the abundance of aquatic vegetation a few kilometres downstream of Mealhada, in the area of Mogofores and S˜ao Jo˜ao da Azenha, where daytime oxygen saturation raised to values above 100%. The water quality improved considerably in this segment of the C«ertima River, pointing to a very fast self-purification during the summer season. Surface waters of the C«ertima were considerably enriched with nutrients just upstream of Pateira de Fermentelos, suggesting that the river can seriously disturb the functioning of this aquatic ecosystem. As far as we know, studies on the trophic status of the lake were never performed, but signs of eutrophication were evident in recent years, due to the large amount of vegetation that periodically develops in the waters. Therefore, further research should complement the present study and be centred on the lake nutrient dynamics and productivity. Finally, the information gathered from this work also demonstrates that more investments must be done in wastewater collection and treatment systems in order to prevent the negative effects of anthropic sources of pollution, to guarantee a sustainable use of water resources in the C«ertima River basin and to minimize the eutrophication risk for the lake of Pateira de Fermentelos.

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