1Urban Water Management

Baltic University Urban Forum Urban Management Guidebook I Urban Water1 Management Editor: Elzbieta Plaza Project part-financed by the European Union (European Regional Development Fund) within the BSR INTERREG III B Neighbourhood Programme. 1 Copyright: The Baltic University Press, 2007 2 Small-scale Water Supply in Urban Hard Rock Areas 4 Bo Olofsson, Royal Institute of Technology and Esa Rönkä, Finnish Environment Institute TODAY’S SITUATION sewage water produced. This new demand has in Water is our most important foodstuff. A grown- turn sometimes led to quantitative and qualitative up person needs at least 2.5 l/day of fresh water groundwater problems, which is clearly evident to- in order to survive. However, we often use 20-100 day in the surroundings of Stockholm, whereas the times more water than we actually need. Although situation is not yet that critical around Helsinki. The water of drinking quality is not necessary for wash- development and municipal planning of these ex- ing purposes, there is a preference for only one sup- ploited areas is strongly dependent on the available ply system in houses due to economical reasons. A water resources and wastewater treatment facilities. large proportion of the water that we use is trans- A commonly asked question is whether the said ar- formed into sewage water and, therefore, the water eas should be incorporated into the district water supply and wastewater treatment cannot be han- and wastewater network, which is normally very dled separately. From a political point of view, the expensive, or whether they should remain based on environmental aspects of water and its quality have local and often individual sanitary solutions. increased over the last couple of years. This fact is evident in the EC framework directives. In Sweden, 15 broad political environmental goals have been AVAILABLE WATER RESOURCES formulated, among which is the goal of achieving Many of the densely populated regions in Scandi- groundwater of good quality. navian countries are located in areas with limited Small-scale water supply in urban hard rock ar- groundwater resources. Some of the biggest cities, eas comprises mainly dug or drilled wells. Sweden such as Stockholm, Gothenburg and Helsinki, are has nearly 400 000 private wells, supplying more situated on hard rock terrain with a large amount of than 1.2 million people (SGU 2003). Most of these bare outcrops, and in the lower parts on soils, mainly are drilled wells constructed after the 1970s. The consisting of till, clay and sometimes sand and gravel. situation in Finland is slightly different. While there Figure 1 shows an overview of the geological condi- are approximately 450 000 private wells (300 000 tions. The areas of bare hard rock stretch from the dug wells and 150 000 drilled wells), the number of surroundings of Stockholm, along the belt of islands drilled wells however are increasing drastically. Pri- (including the archipelago of Åland) to Åbo (Turko) vate wells currently supply about one million people and along the Finnish coastline to Helsinki. in Finland (Lapinlampi et al 2001). Water distribution within cities is based prima- rily on a municipal network. The supply source is ei- THE HARD ROCK AQUIFER ther treated surface water, such as water from Lake In the hard rock terrain, available groundwater ex- Mälaren in Stockholm and Lake Päijänne in Hel- ists only in fractures and zones of fractured rock. sinki, or groundwater that often needs to be artifi- Hence, the fracture type and genesis, the fracture cially recharged. However, areas outside of the cities, frequency, orientation, length and width, occurrence especially Stockholm and Helsinki, usually have a of fracture fillings, fracture surface roughness etc. are mixture of summer housing and permanent housing important factors for water flow. Since the hard rock that often rely on a small-scale water supply based terrain is characterised by a strong geological hetero- on private drilled wells. There is a large development geneity and often-huge investigation costs, it is very pressure on these areas due to the fact that they are difficult to obtain sufficient knowledge about the often located in attractive surroundings. As a con- actual water resources that are available. The yield sequence, pure summer housing is being converted of single boreholes and the storage capacity of the into permanent housing. Higher sanitary standards, area is in reality often strictly dependent on the lo- associated with permanent housing, have resulted cal geological conditions, whereas the groundwater in an increase in the specific demand for fresh wa- resources have to be estimated very roughly from ter and a corresponding increase in the amount of yields of previous boreholes, generalised informa- Small-scale Water Supply in Urban Hard Rock Areas 21 Figure 1. Outcrops of hard rock dominate areas to the east of Stockholm. Generalized geological map modified after SGU. tion based on rock types and sometimes tectonic THE SOIL AQUIFER mapping from air-photos and airborne geophysics The predominant soils in hard rock terrains in the (Figure 2). Geological lineament structures traced Nordic countries are till and clay. Till is a hetero- over long distances are generally steeply inclined, geneous non-sorted glacial deposit, generally rest- whereas the dip of the structures conducting wa- ing directly on the bedrock surface. As a matter of ter into the boreholes are often much flatter. Such fact, 75% of Sweden and 53% of Finland are covered flat fractures can act as collectors of infiltrated wa- with till, whereas in the coastal and wave washed ar- ter through steeply inclined fractures and are much eas, much of the till deposits have been out-washed more common in the uppermost part of the rock. and re-deposited. In the belt stretching from eastern Hydraulic measurements in boreholes using packers Sweden, over Åland to southern Finland, till is today show at least a ten-fold higher hydraulic conductiv- found on the hill slopes and in the bottom of val- ity in the uppermost 100 m of the bedrock than at leys, often covered with sandy beach sediments and deeper levels (SKB 1992). clay. Originally, the local water supply for many of The quantity of groundwater available in the hard the houses in the hard rock areas was obtained from rock terrain is usually very small. The kinematic po- dug wells in such soil deposits. Dug wells have been rosity (flow porosity), which includes the amount of replaced by extensive drilling of wells in hard rock cavities and holes that can actually contribute to the areas because the dug wells tend to run dry during fracture flow, is usually less than 0.05%, although the later summertime and they are vulnerable to be- the fracture porosity in total can be much higher. ing polluted. However, the small soil deposits may This is due to shearing and roughness of the fracture be extremely important as water reservoirs for the surfaces, which force the water to flow on only a bedrock, since their kinematic porosities are 100- part of the surface, usually less than 25% (Hakami 1000 higher than that of the hard rock. Hence, a 1995). Local geological mapping, including meas- decimetre of saturated sand may contain more avail- urements of fracture frequency, type and orienta- able water then one hundred metres of hard rock. tion, is important when estimating available water For this reason, it is extremely important to also resources in such hard rock areas. However, this is consider the composition and stratigraphy of soils seldomly carried out for individually based water during estimations of groundwater resources in hard supply solutions as no single well owner can afford rock terrains. such investigations. 22 Small-scale Water Supply in Urban Hard Rock Areas Figure 2. The heterogeneous groundwater flow in hard rock shown at different scales - a principle sketch. A rough lineament map is made based on topographi- cal maps, air photos and geological maps. A principal geological section crossing one of the lineaments is drawn based on field investigations, such as soil and rock mapping. Detailed fracture pattern analysis in the wells requires extensive well logging and hydraulic tests. The ex- act water flow and flow paths along the single fracture will still be unknown due to the high heterogeneity and surface roughness. Therefore, two closely located wells often exhibit very different yields and water chemistry. heterogeneity of the fractured hard rock; wells that have been drilled too deep and poor sewage water solutions that often include infiltration of grey water. NITROGEN AND BACTERIAS Nitrogen compounds, such as nitrate and ammo- nium, are sometimes found in the drilled wells. The main sources of the nitrogen compounds are commonly: fertilizers from agriculture, airborne nitrogen from traffic and leaking sewage systems. The latter is probably the most important source for groundwater pollution in developed areas. Studies of densely populated islands in the Stockholm archipelago Figure 3. Examples of problems for groundwater quality in hard rock areas show that coliforms and raised concentrations of nitrate are not uncommon in drilled wells due to QUALITY AND QUANTITY PROBLEMS pollution from sewage systems. In comparison, oth- The scarcity of groundwater in developed areas may er studies carried out in developed areas north of in the long-term lead to qualitative and quantitative Stockholm showed that in areas where infiltration problems, Figure 3. Such

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