This Appendix 1 is in response to specific enquiries I have received relating to the Depth and Properties of Water in Wells and Springs around Dulwich
Mineral spa waters in the south of England (of which there are dozens of known sites within and beyond the Effra basin – Camberwell, Stockwell and Bermondsey and Beulah Spas spring to mind immediately) are usually referred to as ‘chalybeate springs’. If you search online under that name you will find some definition and explanation, references to their mineral composition, properties, social history and so on. There are links on Wikipedia to Beulah Spa and Upper Norwood, but alas nothing specifically on Sydenham or Dulwich Wells. Iron is one of the commonest minerals on the planet (along with oxygen and silica) and is therefore one of the main constituents of both rocks and spring water (hence the common orangey or yellowy colour of water, rocks and soils when first exposed to the air), along with magnesium, manganese, potassium and aluminium salts and bases. These are mostly by-products of the chemical weathering of clays – and the main bedrock in our area is, of course, London Clay.
Where did the minerals in the well and spa water, particularly the iron, come from originally? Mineralisation in nature is usually a slow process: minerals require varying time for their solution and for weathering in the rocks to occur, for chemical reaction and interaction to take place, and for their transfer, concentration and precipitation in new locations. Iron minerals in sedimentary rocks and soils seem to be mainly associated with sands and sandstones; the higher iron concentration in some of the springs of London may be derived ultimately from thin siliceous (sandy) bands laid down locally within the London Clay, and well attested in our area of South London (they give character and colour to the local bricks for example), or even as a by-product of the weathering and removal of younger rocks that were laid down on top of the Clay, including the plateau sands and gravels identified at Crystal Palace, and the river terrace and alluvial deposits in the lower reaches of the Effra and Thames basins.
To explain why these mineralised waters are concentrated in particular locations (both spatially within an area and vertically within the rock), we must consider how the water and its dissolved minerals move within the drainage basin. In its simplest form the drainage basin is the means by which water, mainly falling as rain, moves from higher to lower areas by gravity, usually ending up in the oceans, eventually to evaporate and be recycled once more – the familiar Water or Hydrological Cycle. However, it moves in different ways, along different paths and at different rates, and that affects the weathering, transport and delivery of the minerals contained within it.
a) Some rainwater collects on the surface (especially if the soil underneath is saturated), and it may be evaporated quickly back into the air (perhaps leaving a thin deposit of the most readily precipitated salts on the surface, especially in summer), or it may be taken up by plants, animals and humans, and returned to the air through respiration and transpiration. b) Some of that surface water will spread and move sideways under gravity, where it becomes surface runoff, then concentrates into gullies, streams and rivers, carrying its minerals with it and concentrating some of them temporarily alongside and within its channels. This surface flow is generally the fastest route through the basin to the sea, measured in minutes or a few hours in a small basin like the Effra, and is the main cause of local flash floods. c) If the soil is not saturated, and the subsoil is permeable and has pores, cracks and voids in it, then increasing quantities of the rain and surface water will infiltrate and percolate downwards and sideways, travelling approximately parallel to the surface as throughflow . It may be stored temporarily in the soil, or seep back to the surface locally as springs to augment the surface flow, and this is a somewhat slower route through the basin, measurable in hours. d) Most soils and their parent rocks are permeable to a greater or lesser degree, and much of the rainwater will percolate into the bedrock, until it meets either an impermeable layer, or a layer already saturated with water from previous rainfall - which together make up the water table. The height and depth of the water table will vary from season to season, or even over longer periods, according to climate. This groundwater flow still moves water and minerals through the drainage basin by gravity (or by capillary action and hydrostatic pressure), but is usually the slowest and least direct route to the sea, and may take weeks or months, even in a small basin. (e) Finally, if there is a considerable thickness of unsaturated rock deep underground, then any slowly percolating water may effectively be stored almost indefinitely, with its dissolved minerals. It is this water that often allows the lower reaches of rivers to contain water or to continue to flow in prolonged drought. This may be termed baseflow.
From the foregoing, it follows that the quantity and quality (mineral content) of the water may vary considerably in different parts of the drainage basin, and at different depths in the rocks. Many of the older wells are shallow (eg in Dulwich), some are dug to a depth of 100 feet or more (eg at the former Stockwell brewery), while some commercial boreholes may be many times that (eg those that used to feed artesian water from the basement Chalk to Meux's Brewery on the Thames and the Trafalgar Square fountains in Central London).
To help clarify the matter of well and water table depth within the Effra basin further I would also offer the following caveats: 1: Underground water movement to and within the main regional water table rarely involves free movement or flow (as in underground rivers and lakes), except in particularly well jointed sedimentary rocks such as sandstones and limestones, or volcanic rocks, and there are none of those in the Effra Basin. 2: The London Clay is hundreds of feet thick in the Lower Thames Basin, so much water movement is in the form of groundwater and baseflow. Below the Clay is the main Chalk aquifer (storage layer), and much of its water is derived from precipitation on the North Downs and Chiltern Hills that has moved through the Chalk underneath the Clay over many centuries, or even millennia. 3: Water tables are saturated zones or levels within the body of rocks; variations in the character (lithology) and arrangement (stratigraphy and structure) of the rocks will affect groundwater movement and storage, and may produce local (‘perched’) water tables at lesser depths, as has been suggested for water seepage around Dawsons Heights in East Dulwich (picture 10 in the main text), below Eliot Bank and Hornimans in Forest Hill, and in Upper Norwood. Springs occur where the water table level reaches, or is intersected by, the ground surface; wells are where people have dug or bored vertically into the ground to reach the water table below. 4: The historic wells we hear about in our area were generally shallow and always likely to have been comparatively short-lived. Chalybeate well concentrations (such as the cluster of a dozen or more at the former ‘Green Man’ in Dulwich, now 'The Grove') may be as much a matter of chance as due to geological factors (ie where and when they just happen to have been discovered and exploited rather than any special character or levels of concentration). 5: The height, depth and location of local water tables vary according to seasonal precipitation and evaporation rates. Wells and springs may dry up over time when and where ‘draw-down’ exceeds rate of recharge, for whatever reason. In unsaturated free draining rocks the level of the water table may be at a considerable depth beneath the ground surface, possibly even below the altitude of neighbouring valley bottoms, so that water will locally have to move uphill (by hydrostatic pressure) in order to drain away. 6: Watersheds and stream channels are by definition surface topographical features; underground drainage may be directed in a different direction , possibly even via a different stream or basin (as a result of geological conditions, eg strata tilted, folded or fractured at variance with the slope of the ground surface, as commonly happens. 7: Human activity is obviously an important factor affecting the water table: a) Deforestation and land clearance is a main cause of the increased occurrence of floods and landslides throughout the world, since trees and plants take up a lot of water from the soil and lower the water table, intercept rain and slow runoff, and help to bind the soil together and protect the slope from erosion. Modern land planning and management practices take all this into account, but there is still much to be learned concerning the complex interactions within drainage basins, especially urban ones like the Effra.
b) Urbanisation covers much of the ground with impermeable surfaces, thus increasing runoff (usually into drains rather than soakaways), and thereby lowering water tables, as has happened widely in SE England; hence the importance of recent 'greening' policies in the London Boroughs. In East Dulwich the building of the Dawsons Heights Estate seems to have reactivated springs and may be the cause of basement seepage in surrounding streets (see picture 10 in main text).
c) Urbanisation also increases the risk of contamination and pollution, hence the need for sewage and water treatment schemes like those of Joseph Bazalgette in 19th century London. d) Land drainage schemes and flood prevention measures (eg recent projects in Dulwich Park, Belair, and in SE24) aim to decrease runoff and flood risk by controlling water movement through the basin.
8: Rainfall intensity affects the capacity of the land to absorb water: heavy, intense rainfall causes the pore spaces in rocks and soils to fill up more quickly than they can drain away. So the increased incidence of storm events as a result of global warming (itself largely a result of human activity) will have a major impact on water tables and flood risk, as regular walkers in Dulwich and Sydenham Hill Woods, and residents further downstream are now experiencing.
(Pictures 10 and 11 in main text: Recent flooding at the former confluence of Effra tributaries in Herne Hill: anti-flooding measures have just been completed in Dulwich to alleviate the effects of events such as these.)