Recharge systems for protecting and enhancing groundwate

TOPIC 1
River/lake bank filtration and pond infiltration issues / Recharge systems 35
TOC
(mg/l)
DOC
(mg/l)
Channel 17,4 10,4
Pregolya Upstream 10,8 9,7
West Shallow Lake 10,6 9,7
East Gravel Pit 9,8 9,1
South Gravel Pit West 8 7,4
South Gravel Pit East 7,8 7,4
South Bank Well 1 7,3 6,7
South Bank Well 2 5,9 5,6
OC-Response
21,5
16,5
11,5
6,5
1,5
Polysaccharides
Humates
Decreasing
molecular size
0 20 40 60 80 100 120
Time (min)
Channel East Gravel Pit Pregolya Upstream
Figure 3. TOC, DOC and LC-OCD profiles of surface waters and groundwater
at the Eastern Wellfield. Samples taken on June 18th 2004
Determinants of the observed level of organic matter
The model and temperature observations suggested different degrees of contact with the channel throughout
the wellfield. From the point of view of achievable quantity, this is unlikely have a detrimental effect because of the
high transmissivity of the aquifer and the possibility of recharge from several other surface water bodies. However, a
complex pattern with regard to water quality is likely to emerge. In the areas with good contact to the channel,
average filtrate travel times may be as low as days or weeks, whereas travel times of only a couple of months may be
encountered in other parts. Grünheid et al. (2004) have shown that optimal degradation of organic matter may
require filtrate travel times of at least 2 weeks under aerobic conditions and over 3 months under anaerobic conditions.
The good correlation of fairly high-frequency variations (order of magnitude of weeks) in raw water from the North
and South bank rows of wells suggests that the overall determinant for observed variation lies in a common source,
which the model implies must be the surface water, mostly the channel. The low significance of local precipitation
precludes that ground surface infiltration should be having a significant effect. The historical data indicates an
apparent consistent minimum value of raw water oxidisability of approximately 3,5–4 mg/l, which is close to the
Russian drinking water standard of 5 mg/l. Fairly large variations take place, with values apparently reaching up
to 7–10 mg/l at certain times. Unfortunately, no measurements outside the wellfield area exist to give a conclusive
picture of the background level of oxidisability/organic matter. Moreover, analytical inconsistencies between surface
and raw water analyses make it difficult to determine the degree to which variation in the surface water are presently
translating into variations in the raw water.
The breakdown of the dissolved organic matter in the surface water shows that it is dominated by humates, with a
minor component consisting of polysaccharides. Interestingly, the humate profiles are quite smooth and narrow,
without extended ridges representing intermediary states or light molecular fractions. This may be an indication of
a very stable nature, which may in turn be an indication that the humate is mainly of geogenic origin. Moreover, the
profiles are similar. The ubiquitous presence of peat and drained areas throughout the floodplain would be a likely
source. The DOC measurements show an increasing trend in a Northerly direction towards the Pregol River, albeit
there appears to be a separate source of organic matter in the channel at the time of observation in June of 2004 as
seen in the difference between TOC and DOC as well as the LC-OCD profiles.
Visual inspection and historical water data indicate an annual algal bloom that lasts from June to September,
depleting the channel of Nitrate. Sulphate-reducing conditions seem to exist at the bottom of the channel in
summer. Moreover, staff at the wellfield report that there is a layer of suspended sludge at the bottom of the
10 – 16 June 2005, Berlin ■ 5th International Symposium ■ AQUIFER RECHARGE ■ ISMAR 2005