Comments on Appendix 9.1A (Hydrogeological Risk Assessment)
prepared for the UKOG planning application for oil extraction at
Markwells Wood in West Sussex.
1. The conceptual model used to assess the risk of pollution of public water sources by
this proposed development is based upon a number of key elements listed in the NonTechnical Summary. Several of these elements conflict with information provided
within the main text of this report and within the Hydrogeological report prepared for
the Markwells Wood Watch group, as well as with other unmentioned published
information relating to the local geology.
2. The well site is located on the southern side of a dry chalk valley which links to a
larger dry chalk valley known to be on a significant major groundwater flow path to the
Havant and Bedhampton springs. Although the text discusses the possibility of this dry
valley also being a zone of unusually high groundwater transmission for the area,
because of fault induced rock mass fracturing and associated karstification, it is
considered to be insignificant in the risk assessment. This is surprising as the depth and
nature of the fluid loss recorded during the drilling of the existing well could be related
to deep karstification along a fracture zone. An alternative, arguably less plausible,
reason related to variations in the lithology of the Chalk formation is provided. No
ground investigations or hydrogeological tests have been carried out to determine the
lateral extent of rock mass fracturing and karstification at this location.
3. The statement that there is ' limited' potential for solution enhanced weathering to
develop is misleading because these processes develop over thousands of years and it is
the existing karstification of the chalk that is more relevant in this appraisal.
Consideration should be given to the possibility of active karstification of the chalk
having taken place in the past, in the Markwells Wood area, when the chalk was
covered by up to 12 metres or so of Reading Formation sand and clay.
4. The statement that there are no dolines in the Markwells Wood area is almost
certainly incorrect. The national data bases for dissolution features mentioned were
originally developed before the advent of LIDAR imaging and may well be out of date for
the Markwells Wood area. The hydrogeological report prepared for Markwells Wood
Watch identified many of the surface depressions in the Markwells Wood as being
either dolines or as dolines subsequently quarried. Within the Stansted Estate,
immediately to the south of this area, LIDAR imaging has revealed a high density of
circular surface depressions, many of which have the appearance and attributes of
proven dolines elsewhere.
5. Mention is made of infilled dolines in the Markwells Wood area but they have been
incorrectly considered to be insignificant in the risk assessment. The conventional aerial
photograph of the Markwells site,shown in the EIA , indicates many circular features
which at other similar chalk sites have been shown to be infilled dolines. The clay-with flints cover could be less of a hydraulic barrier between the ground surface and the
chalk than anticipated because of loosening and voiding resulting from ground
subsidence. The tracer tests and ground investigations carried out at the A3M Hazelton
Interchange dolines ,( McDowell et al QJEGH 2008) showed that the infill soil can be
very permeable.
6. No consideration has been given to the possibility of polluted water losses reaching
the chalk aquifer as a result of rock mass fracturing, particularly if high fluid pressures
are adopted.
The drilling notes presented by Northern Petroleum to assist in the selection of the
existing well site mention these risks in relation to a major East - West fault through
Forestside. This fault appears to have been indicated by 2-D seismic reflection surveying.
No 3-D seismic reflection or seismic refraction surveying has been carried out at the
chosen site to investigate fault zones at different depths and orientations.
Peter McDowell, C Geol. F.G.S.
Engineering Geologist
21 March 2017