Towards a Baltic Sea Region Strategy in Critical ... - Helsinki.fi

CHAPTER VI: WATER
The complicated geological structure of the First Salpausselkä formation results in
complicated groundwater conditions. Permeability varies in the Hanko aquifer due
to the changes in the grain size and thickness of the different layers. The structure
of the aquifer in the northwest part is quite different to the southeast part.
According to drillings, the sand and gravel layers are varying with clay and
silt layers (Figure 6). In many places the geological stratigraphy from ground level
downwards is: first sand and gravel layers with good water conductivity, then silt
and clay layers with weak conductivity, followed by sand and gravel layers with
good conductivity. According to Breilin et al. (2004) the groundwater level in the
study area varies from less than 2 meters to more than 15 meters below surface.
The groundwater table is at its deepest level in two different parts in the northern
and southern part of the formation (Figure 7). Right by the Baltic Sea coast the
groundwater table is near the ground surface. Generally, the groundwater table is
less than + 2 m a.s.l. (N 60 ), only in the centre part of the formation the
groundwater table is + 4 - +6 m a.s.l. (N 60 ).
Water supply
There are four separate public groundwater works in the studied aquifer (Figure 7).
In 2003, the total water use of the town of Hanko was on average 4 630 m 3 /d. The
combined pumping volume from four water works in the study area was 1 430
m 3 /d, which contributed about 30 % of the Hanko’s water production, even though
the yield and the capacity of this formation are much larger than that now used.
The name of the oldest water works is Hopearanta in the north part of the
formation, built in 1907. This is one of the oldest water works in Finland. The
pumping rate of this water work is of 700 – 800 m 3 /d. The water work consists of
three different wells and the total pumping permission is 1 200 m 3 /d. The recently
defined catchment for the wells is about 1 km 2 .
The water works of Mannerheimintie is located in the south part of the
aquifer quite near the town. The pumping permission is 720 m 3 /d and the daily use
in general is about 320 m 3 /d. However, at the moment this water work is not in
use. The current estimation of the size of the catchment is about 0.5 km 2 .
The water work of Ampumarata has a pumping permission of 2200 m 3 /d, but
it is not possible to get that much water from the two wells in the area. The water
quality is not good enough in this water works, the problem being too high iron
and manganese contents. The water use is now only for the irrigation of the sports
area.
The water works of Furunäs has been closed since 1982 because of a leakage
from the sewer water system of a nearby pharmaceutical factory.
In addition to the water works, there are some private wells in use, mainly for
irrigation. There are also many groundwater observation wells, mainly in the
vicinity of the water works but also elsewhere in the area of the aquifer. The length
of underground water work system network in Hanko is in total 155 km.
Groundwater quality
The Hanko aquifer has been burdened by different activities and factors, and the
water quality has decreased in places. The water quality at the groundwater works
is regularly sampled by the health authorities and the staff of water works. The
analyses and measurements are according to the Act of Water quality requirements
NORDREGIO REPORT 2007:5 201