The Role of Groundwater in the Acidification of the Hydrosphere - Examples from Small Catchments in the Bohemian Massif
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Z . HRKAL, J. BUCHTELE, E. TlKKANEN, A. KA pA H O s J. SANTRUCEK N GU -BULL 439, 2002 - PA GE 99 The role of groundwater in the acidification of the hydrosphere - examples from small catchments in the Bohemian Massif ZBYN EKHRKAL,JOSEF BUCHTELE, EEVATIKK ANEN,ASKO KApAHO &JAROMIRSANTRUCEK Hrkal, Z., Buchtele,J.,Tikkanen, E., Kapyaho, A.& Santrucek,J. 2002:The role of groundwa te r in the acidifica tion of the hydrosphere - exam ple s from small catchment s in the Bohem ian Massif.Norges qeoloqiske undersokelse Bulletin 439, 99-105 . The aim of the pre sented study was to assess the im pact of the groundwa ter on the degree of acidificati on of sur face w aters in small catchm ent s. Quantit ati ve hydr ogeological analy sis based on th e result s of the SACRAMENTO hydrolog ical mo de l was aime d at th e assessment of th e contribu tio n of baseflow repr esentin g th e groundwater dis charg e in the tot al runoff. Co mparison of single pH measureme nts in surface wate r wi th the corr espondin g actual and mo delled w ater discharge was used as th e info rmatio n in th e qu antitative part of the data processing. The obtai ned result s showed differen t prot ective pow ers of th e aq uifers against acidifica tio n,a conspicuous decrease in the soil buffer capac ity of th e Krusne hory Mts. and a significant influence of th rou gh fall precipitati on s on the gr oun dwater qu alit y. Zbynek Hrkal,JosefBuchtele, Jaromir Santrucek, Cha rles University, Dept. ofHydrog eology, Czech Republic. Eeva Tikkanen, Asko Kapyah o, University ofHelsinki, Fin land. Introduction ity of surface water and groundwater could have been The Czech Republic is one of several countries where th e obta ined only from th e monitoring of small experimental effects of anthrogenic atmospheric acidification first began catchments. However, in small catchme nts in th e Czech to reveal th emselves in th e early 1960s. However, the mass Republ ic, as in other countr ies, th e contribution of ground extinction of forest vegetation, first in the highe r parts of the water discharge and quality to surface water was most ly Krusne hory Mts.and later in th e Jizerske hory and Krkono se neglected. Comparisons were made between th e amount Mts., demonstrated the tru e extent of acidification.This pro and quality of precipitation on the one hand, and the same vided conclusive evidence of th e widespread nature of the for runoff on th e oth er hand, without considering the vari acidification, but some doubts still prevailed about th e ability of the ground wat er component in th e surface water. degree of acidification of groundwat ers. Some regional In recent years, an investigation has been sponsored by studies (Hrkal 1992,Hrkal & Fottov a 1999) reported evidence Grant 11310005 of th e Min istry of Education of the Czech of progre ssive acidification of shallow, near-surface aquifers Republic 'Material and energy flows in the uppe r parts of th e in the crystalline bedrock compl exes which revealed a con Earth', and this has been carried out within the 5th siderable decrease in alkalinity leading to th e complete dis Framework Programm e sponsored by the Europ ean Union . appearance of bicarbonates,decrease in pH,and an increase The LOWRGREP project 'Landscape-use optimisation with in AI and Be concentrations. Whilst the quality of surface regard to ground water resources protection in mo untain waters is affected by atmospheric deposit ion directly and hardrock areas' was intended to assess the contribution of im mediately, the impact of rainfall on th e chemical composi groundwater im pact on the deg ree of acidificat ion of sur tion of groundwate rs is delayed. Owing to th e contact of face waters. ground water with th e bedrock enviro nment, and also pri marily with soil strata, th e ground water enjoys a degree of Method of data processing potential buffering against acidification, and consequently The selection of test sites was th e first step of the wor k. For may play an important role in th e resulting chemistry of sur this reason, data from th e experimental small catchment face water discharge. However, th e proportion of ground network GEOMON (Fottova 2000) have been used. The water in the total discharge varies considerably in the course degree of acidification in single catchm ents is controlled by of the year, and consequently a similar variation can be seen groundwater to a varying extent. The comparison of pH in the degree of its impact on th e final chemical composi input, represented by atmospheric precipitation and tion of surface waters. throughfall, and the output, represented by pH and alkalin More precise data on the relationship between th e qual- ity of surface runoff, was used to assess the deg ree of resis- NGU -BULL 43 9, 2002 - PAGE 10 0 Z. HRKAL, J . BUCHTELE, E. Tl KKANEN, A. KApA HO e J . SANTRUCEK I.' 18' assessment is based on the median from these arithmetic averages.Castany et al.(1970) are of the opinion that reliable ,....""~.....",,,;;l 00 km N results can be obtained from a series of measurements over = * Poland a minimum of 10 years,whereas currently the longest period of monitori ng extends only over 5 years. Consequently, the presented results should be viewed with some caution. : ~Prague To incorporate a mo re precise baseflow assess ment , the CZECH R£J>UB LIC SACRAM ENTa soil moi sture accounti ng model has been used (Burnash 1995).This model, in combination with the Anderson snow model (Anderson 1973),enables continuous Slovakia simulations to be carried out over a period of several years. For simulations of the annual rainfall -runoff process, the daily time series are usually used.This is a conceptual water o Krusne hory Mts. • Jezeri catchment balance model with lumped inputs, with the possibility for ~ Sum ava .MIs. • Lysin. and Pluhuv Bor catchment in the Slavkovsky forest its implementation in a semidistributed mode. "* Na Lizu catchmen t There are altogether six runoff components generated by the mod el: Fig. 1. Location of the test sites. DIR - direct runoff, from those part sof the basin whi ch become impervious after saturation, trvrty of ind ividua l catchments against acid atmospheric IMP - the runoff from the permanently impervious part deposition. It was found that the greater the difference of basin, bet ween th e pH of atmospheric water and in runoff and a SU R - surface runoff, higher alkalinity in surface water,the greater was the buffer IN - interflow, ing capacity of the hydrogeological environment and conse SUP _. supplementary baseflow, i.e., essentially the sea quently its resistance against acid deposition. sonal component of baseflows, However,the basic information concerning the qual ity of PR M - primary baseflow, i.e., the long term part of base gro undwaters was still missing when assessi ng the degree flow. of grou ndwa ter impact on th e final chem istry of surface The daily precipitation and temp erature data typical for water. Mo nito ring of th e groun dwater quality within the th e catchment area have been used as the input data. The LOWRGREP project comm enced in two small catchments model runoff simulation was then calibrated to the mea (Jezeri and Na Lizu, see Fig. 1) as late as in the beginning of sured surface runoff data and the model assessment of the the year 2001.The first results from these localities revealed baseflow compa red with the minimum runoff data. that th e ground water has a more alkaline reaction that the The quantitative part of the data processing was fol surface water and, consequent ly, the gro und water should lowed by analysis of the groundwater impact on the final restrain the impact of 'acid flash floods ' on the pH of surface chemical compos ition of the surface flow. The following waters.On the other hand, it is obv ious that the chemist ry of meth od was used to express the impact of groundwate r on groundwater dep ends closely on the type of host rock envi the chemistry of the surface runoff. The hydrogram of the ronment. ln other words, the pH of groundwater confined to total runoff has been separated into th ree parts. One part granite will differ from th at, for example, in serpentinite. includes th e period when th e ground water discharge con Therefore, th e chemical composi tion of surface water does stitutes at least 50% of th e total runoff and when the not depend only on th e quantitative parameter, i.e.the pro groundwater chemi stry has most effect on the quality of portion of groundwater in th e total runoff, but also on the surface waters.The op posite case is represented by a period quality of th e groundwater. when the contribution of groundwater discharge in tota l The work was carried out in two main stages; firstly, a runoff decreased below 25%. In such a case, the quality of qualitative assessment, which in turn provided data for a atmospheric water plays a decisive role in the quality of the quantitative analysis. The quantitative hydrogeologica l tota l runoff. The period during whic h the contribution of analysis was aimed at an assessment of the contribution of groundwa ter discharge in total runoff varied between 25 baseflow, representing th e gro und water discharge in the and 50% was considered a mixed period in wh ich none of totaI runoff.