Groundwater

The year in brief

Rainfall over the outcrop areas of most major aquifers in 2001 was generally close to the 1961-90 average, albeit appreciably above avarage across a substantial proportion of the eastern Chalk. Despite above average evaporative demands, overall infiltration totals were also well within the normal range. However, the overall range in groundwater levels in 2001 was exceptional, a reflection of both antecedent aquifer recharge and the very unusual distribution of recharge through the year - remarkably high totals characterised the January- April period but, thereafter, replenishment was very modest, notably so in relation to the seasonal average during November and December.

In all but the fastest responding aquifer units, groundwater levels reflect recharge patterns over several winters. An important influence on levels in 2001 was the above average rainfall in many aquifer outcrop areas over the winters of 1997/98, 1998/99 and 1999/00 and, particularly, the unprecedented rainfall over the lengthy 2000/01 recharge season. In assessing the impact of this rainfall on groundwater levels it is important to recognise that its relation with aquifer recharge is not a linear one. Evaporation losses have a proportionately much greater impact on recharge in the drier eastern lowlands than in the wetter west. As a consequence, in eastern England, where the Chalk outcrops extensively, a 20% increase in winter rainfall can result in a doubling of aquifer recharge. This non-linearity has been well demonstrated in the recent past.

Entering 2001, groundwater levels throughout most of the UK were close to, or above, previous maxima and - with a proportion of the late-2000 infiltration still to reach the water- tables in many areas - set to rise further through the late winter. The 2000/01 recharge season was exceptionally protracted, exceeding 30 weeks in most regions, and the magnitude of replenishment was without modern parallel (see Table 1). Record groundwater levels continued to be registered well into the spring of 2001 - and later in the slowest responding aquifer units (including large parts of the Permo-Triassic sandstones). Such areas aside, brisk groundwater level recessions became established during the early summer and continued until year end in many eastern outcrops.

Figure 1 shows 1997-2001 groundwater level hydrographs for a selection of index wells and boreholes throughout the UK (see map for the location of the index sites). Five-year plots have been used in recognition that groundwater levels in many areas show considerable persistence. The 5-year format also allows the outstanding range in groundwater levels over the recent past to be illustrated. The groundwater level trace is shown together with the monthly maximum and minimum levels for the pre-1997 record. The normal seasonal variation in levels is clearly evident in the hydrographs for most sites; other common features include the erratic recharge pattern over the 1999/2000 winter half-year and, most notably, the extraordinary seasonal recovery in late 2000. In overall resources terms, an important contrast can be drawn between most of the last four years - when groundwater levels have been generally above average, and the depressed levels of the 1995-1997 period; in the summer of 1997 groundwater levels were historically depressed across much of the UK.

The majority of observation wells and boreholes for which data are held on the National Groundwater Level Archive monitor the natural variation in levels. However, in parts of the UK groundwater levels have been influenced, sometimes over very long periods, by pumping for water supply or other purposes. As a consequence, the local or regional water-table may become substantially depressed. For instance, contemporary levels at a number of boreholes in the Permo-Triassic sandstones of the Midlands are indicative of a significant regional decline. In London, increasing groundwater abstraction through the nineteenth and the first half of the twentieth centuries led to a 70-metre decline in groundwater levels in the Trafalgar Square borehole. Since the 1950s, a much reduced abstraction rate has resulted in a recovery of around 30 metres; levels rose by 1-2 metres a year through the 1990s (Figure 2) but the rise since 1999 has been much more modest with relatively stability characterising level variations through 2001. Rising groundwater levels have also been reported from other conurbations; leakage from water mains is considered a significant factor in some cases. The implications of rising groundwater levels extend beyond the potential improvement in water resources that the rise represents. Groundwater quality may be adversely affected as levels approach the surface and a number of geotechnical problems may result, for instance the flooding of tunnels and foundations.

Groundwater levels through the year

Following record recharge rates during the last three months of 2000, abundant infiltration continued through January as all aquifer outcrop areas remained saturated. Some brisk declines from the remarkable groundwater levels in December were recorded (e.g. in the Carboniferous Limestone of Derbyshire) but, generally, levels continued to rise. Overflowing wells and boreholes were commonplace in many parts of the Chalk outcrop, in southern England especially. Water-tables reached the surface in many 'dry' chalk valleys producing surface flows at previously unrecorded levels and persistent 'clearwater' flooding was experienced in many areas.

Rainfall in February was around twice the average in parts of East Anglia and the associated heavy recharge produced further rises in groundwater levels throughout much of the eastern Chalk. In Kent, the Little Bucket borehole overflowed - thought to be for the first time (in a 40-year series) and many observation wells reported new maximum levels (for any month). Initial analyses indicate that overall resources in the Chalk (the most important aquifer in water supply terms) were greater than at any time for which directly recorded level data are available. Unprecedented groundwater levels were also reported for the Permo-Triassic sandstones and Magnesian Limestones - and for most minor aquifers in eastern England (e.g. the Norfolk Crag and Essex Gravels). Following winter recharge totals that were three or four times the long term average in much of southern and eastern England outflows from springs were of an unprecedented magnitude - and, in many areas, still increasing.

Access restrictions due to the outbreak of Foot and Mouth disease severely disrupted the collection of groundwater level data in March when infiltration rates were again very heavy. Rapid drainage from high level springs and seepages reduced groundwater levels in some more responsive aquifer units, but generally levels remained historically high - well above pre-2000 maxima across much of the Chalk. Levels in the deep and slow-responding Therfield Well (near Royston), reached their highest since the First World War. Locally (e.g. in the upper Pang catchment, Oxfordshire) pumping was undertaken to help reduce groundwater levels in areas subject to significant flooding. Despite such measures, 'clearwater' flooding remained common and persistent - prolonged basement flooding and transport disruption was widespread in southern and eastern England.

By April, during which recharge rates were again exceptional, previous groundwater level maxima were exceeded across many outcrop areas (Figure 3). Pre-2000 maxima were eclipsed by margins which approached the average annual range across parts of the Permo- Triassic sandstones outcrop (e.g. in the Midlands). At Weeford Flats, where the borehole was dry in early 1999, levels rose briskly through April and were approaching the long term maximum in a series from 1966. In the Chalk, groundwater levels at Stonor, in the Chilterns, peaked around four metres above the pre-2000 maximum and extremely high spring outflows were maintained in many areas.

Below average rainfall in late April and May, together with accelerating evaporative demands and a corresponding rapid increase in soil moisture deficits signalled the end of the most remarkable recharge episode on record for the UK. This heralded a decline in groundwater levels - but its pace was very dependant on aquifer characteristics. In the slow responding Permo-Triassic sandstones and in parts of the eastern Chalk, levels remained above pre-2000 maxima for many further months and clearwater flooding was experienced well into the summer.

June was also dry and notably high soil moisture deficits characterised most areas at month- end. Correspondingly, infiltration was restricted to very localised storm events. Steep groundwater level declines were reported for responsive aquifer units (e.g. the Carboniferous Limestone of Derbyshire) but the large volume of water percolating through the unsaturated zones above many water-tables continued to generate groundwater level increases, in the Permo-Triassic sandstones especially. Overall groundwater resources for England and Wales during the early summer of 2001 were healthier than is normally encountered in the late winter.

The easing of Foot and Mouth restrictions in July helped confirm the contrasting responses in different aquifer types to the dry late spring and early summer. Levels in the western and northern Chalk (generally more fissured and more responsive than in the eastern Chalk) had returned to within the normal summer range; this was true of much of the Carboniferous and Jurassic Limestone outcrops also. However, throughout much of the Permo-Triassic sandstones aquifer new period of record maxima were being established (e.g. at Heathlanes) whilst previous monthly maxima were still being eclipsed in the Chalk during August (e.g. at Stonor and Yew Tree Farm).

By September however recessions were well established in almost all aquifer units and by month-end levels in most limestone and some Chalk observation wells and boreholes had fallen to within the normal seasonal range. Sustained rainfall in October - which favoured the outcrop areas of the major aquifers - produced a steep decline in soil moisture deficits and initiated the seasonal recovery in groundwater levels across much of the country late in the month. In a few Chalk outcrops (e.g. in Dorset), the recovery began from a low base but in the eastern Chalk and across many Permo-Triassic sandstones outcrops it began with groundwater levels close to the October maxima. This gave rise to concern that even with 2001/02 recharge totals within the normal range there was a significant risk of further groundwater flooding through the winter of 2001/02.

Parts of eastern England excepted, soils remained close to saturation through November but - fortunately from the flood risk perspective - rainfall totals were well below average and, as a consequence, the groundwater level recoveries gained little momentum. In most outcrop areas December was even drier resulting in a stalling of the groundwater level recovery in all regions. In parts of the Chalk levels remained close to those of the preceding summer. The year ended with a very spatially diverse pattern of groundwater storage (Figure 4). Below average levels characterised parts of the western Chalk whilst throughout parts of the Permo- Triassic sandstones outcrop levels remained above pre-2001 maxima.

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