Conceptual groundwater modelling studies undertaken prior to construction of the Cardiff Bay Barrage, identified that impoundment of the Bay could have a potential impact on the groundwater levels underlying south Cardiff.

The Cardiff Bay Barrage Act 1993 obliged Cardiff Bay Development Corporation to monitor groundwater levels for a period pre-impoundment, and thereafter post-impoundment until 2019, so that the impact of impoundment on groundwater levels could be determined.

The commencement of impoundment was 4 November 1999.

The operation and management responsibilities for Cardiff Bay Barrage were passed to Cardiff Harbour Authority on 1 April 2000.

In 1995, a monitoring system was established to measure groundwater levels in four hydrogeological units both pre- and post-impoundment of the Bay. The system has expanded over the years and currently comprises 236 groundwater level monitoring instruments (piezometers) installed in some 196 boreholes distributed over an area of approximately 15km2.

At present, piezometers and associated equipment from companies such as GE Druck, Solinst Canada Ltd, Trafag AG and OTT Hydrometry Ltd. are installed in the system. Each monitoring instrument records groundwater levels onto dataloggers at intervals of 30 minutes, mainly in the made ground, estuarine alluvium and gravel strata.

Other instruments record water levels in the rivers Taff and Ely, the Bay and the tide level in the Severn Estuary outside the Barrage. The Harbour Authority maintains its own weather station which records rainfall, atmospheric pressure and temperature.

These statistics make the Cardiff Bay Groundwater Monitoring system  the largest urban system of its kind in the world.

The Groundwater Monitoring & Control Team has responsibility for the downloading, processing, verification and storage of the data from these monitoring stations and ensures the continual operation, calibration and maintenance of the monitoring system.

All dataloggers are normally visited routinely once a month to download data and to inspect the condition of the monitoring equipment and chambers. Inspection of the data identifies the operational status of the monitoring system and actions are undertaken to deal with any instrument “failures.” The aim is to evaluate instrument performance and enable any problems or malfunctions to be investigated and rectified as quickly as possible.

Full recovery of reliable data from individual instruments is not generally achieved for a variety of reasons. The main reasons for incomplete recovery relate mainly to specific instruments that are either removed for safekeeping during nearby construction works or damage to the sensitive electronic equipment caused by ingress of rainwater into the chamber. A target of 95% minimum data recovery for each downloading visit has been established.

All field recordings are transferred and stored in a database from which graphical presentations of the data are then produced.

The amount of data transferred to the database is about 0.5 gigabytes every year, possibly making the Cardiff Bay Groundwater Monitoring database one of the largest databases of its kind.

The collected monitoring data is summarised in reports produced on a six monthly basis which are then reviewed by an independent hydrogeology expert, Professor John Lloyd. His comments are reported to the Harbour Authority and the reports are then made available to the public.

The following reports are available from the Harbour Authority:-