Artesian blister wetlands, the intersection of geomorphology and hydrogeology

Groundwater and Geomorphic Processes
Many geomorphic processes (such as slumps and landslides) involve groundwater, but this invisible component of the hydrologic cycle does not always receive the attention or 'respect' it deserves. In the last 100 years, world demand for water has multiplied by six, twice the rate of population growth over the same period. According to a recent analysis (Colwell and Sasson, 1996), the scarcity of water in specific areas will become the second most pressing concern in the 21st century (the first is population growth). This water crisis and the concern for the environmental impact of global climate change have spawned increased study of groundwater systems.

It is also becoming clear that groundwater plays an important role in many geomorphic processes and more can be learned by studying the two systems together rather as separate entities (Higgins, 1990; Brown, 1995; LaFleur, 1999). This is particularly true in arid regions such as East Africa where droughts have increased in the last several decades and understanding groundwater systems may help improve quality of life (Kayane, 1997; Buckle, 2005). Groundwater-fed springs and seeps occur even in arid to semi-arid regions (Thompson and Hamilton 1983).

The most common geomorphic features associated with groundwater flow are discharge wetlands that are present in areas of low ground such as lake margins, river flood plains, and deltas where the water table intersects the land surface (Mitsch and Gosselink, 2000). Other groundwater-fed wetlands are associated with impervious materials (perched wetlands) or linked to geological structures (faults or bedrock fracture systems) that are conduits for groundwater flow (Figure 1). Artesian wetlands, or spring mires (Moore and Bellamy, 1974), may develop where water discharges under hydraulic head (pressure) (Shedlock et al., 1993; Ashley et al., 2002)(Figure 2).

Artesian Wetlands: Geology and Geomorphology
More than a dozen artesian wetlands (Figure 2) occur within a large spring and wetland complex (1.3 km²)(Figures 3 and 4). The mounds are aligned along north-south system of fractures near the contact between the volcanic bedrock and sediments that are silty clay (Figure 4). The springs are nested into an elongate parallelogram defined by fractures oriented NE-SW (dashed lines) and another system oriented NW-SE solid lines (Figure 5). The site is an alluvial surface (Loboi Plain) just north of Lake Bogoria, Kenya (Figure 3). The mounds are situated near a major northeast-southeast bounding fault to the rift valley.

The artesian springs are completely capped with dense fibrous root mat or wetland vegetation that is strong enough to hold a person, but the surface wobbles and is pushed down several centimeters when stepped on (Figure 2). The arched semi-permanent vegetation cap appears to be buoyed up by slow artesian flow oozing out continuously. These unusual features have been called artesian blister wetlands. The flowing water feeds a ring of sedge (Pycreaus mundtii neesand Cyperus laevigatus)and grass plants that encircle the mound covering up to 50 m² (Muasya et al., 2002) (Figure 2a). Individual mounds are composed of organic-rich clay that contain diatoms, cyanobacteria, and green algae (Ashley et al., 2002; Owen et al., 2004). The mounds are ~15 m wide; 1-2 m high and have a central water blister that may be up to 1-1.5 m wide with a volume of < 1 m³.

Hydrology
The groundwater system is ultimately sourced by rainfall. Precipitation (P) in the region is ~700 mm/yr and evaporation (E) ~2800 mm/yr. Using basic principles, rainfall that is not evaporated nor utilized by plants (transpiration, T) enters the ground in the recharge region and flows down slope under the influence of gravity or hydraulic head (Figure 1). Groundwater is inherently protected from evaporation and thus buffered against short-term changes in climate. The water moves slowly (m/yr) through bedrock and the local geology (structure and lithology) determines the path and the locations where flow discharges onto surface (Marshak, 2004) (Figure 1). With regards to the water source for the blister wetlands on the Loboi Plain, precipitation infiltrates the thin soil and fractured bedrock of the adjacent volcanic highland to the east and moves downhill toward the rift valley.

The Model of Blister Wetland Formation
The model proposed is that fractures in the bedrock intercept the subsurface drainage and focus flow into "pipes", creating a weak artesian flow (Figure 2b). The site of up flow (for a mound) is likely at the intersection of fracture system. The mounds lie along a north-south trend suggesting they are following fractures (Figures 4 & 5). Continuous flowing water balances or exceeds the high evaporation rate in this low latitude setting and supports year-round growth of wetland vegetation. The mound can only build up to the piezometric surface (the imaginary surface to which groundwater rises under hydrostatic pressure). The water is fresh and used by local community for household use and livestock.

The wetland mounds likely form by a positive feedback process that is initiated by the continuous flow of water, which in turn, supports lush vegetation. The wetland vegetation shelter diatoms, cyanobacteria, and green algae and provides a trapping mechanism for sediments washed in or blown to the site (Figure 2b). The mound grows upward by accretion of mineral and organic matter supported by the flowing water. These amazing habitats thrive in a semi-arid region and preliminary dating suggests that they have existed for several hundred years (Goman et al., 2010). The blister wetlands serve as an excellent example of the interdependent nature of geomorphic and hydrogeologic processes in creating this unique landform.

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