Introduction:

The hydrology of the Tularosa Basin has exerted fundamental controls on the evolution of the White Sands Dune field. These controls have worked in several important ways. (1) through provision of evaporites to the basin by ground and surface water flow, including the gypsum that comprises the dunes; (2) by regulating sediment supply to the dunefield by the action of a nearsurface groundwater table; (3) by controlling vegetation type and abundance through changes in salinity and amount of available moisture due to seasonal or longer term fluctuations (4) by the action of water and evaporites in producing the many and varied sedimentary features in the depostional system. This chapter summarizes briefly the characteristics of the ground -water and surface water systems in the basin that are of most importance to the history and current status of the White Sands dune field. In this summary we rely greatly on Bedinger, et. al. (1986) whose team completed a rather detailed modelling study of ground and surface water movement in the basin.

Hydrogeologic Units:

Bedinger, et. al. (1989) recognize a number of hydrogeologic units in and around the Tularosa Basin including mainly partly consolidated basin fill and consolidated rocks, most of which outcrop in the San Andres and Sacramento Mountains on the basin margin (See Figure 1-4 for summary of geology of the basin). Much of the basin fill is of Miocene and younger age, including the Santa Fe Formation and some basaltic rocks such as the Malpais lava flow at the north end of the basin. The basin fill has its origins in the Rio Grande River, which once supplied sand and gravel to the Tularosa Basin. Above this basin fill lie lacustrine and fluvial sands and silts of Pleistocene to Recent age that are several thousand feet thick in the depocenter of the basin. These deposits have a basin-centered pattern, with rather porous and permeable fluvial sands and gravels from alluvial fans around the edge of the basin, and much less porous and permeable silty or evaporitic facies in the lowest, central portions of the basin. Table 3-1 shows the basic geohydrologic units of Bedinger, et. al.(1989) and their associated hydraulic gradients and conductivities.

Table 3-1 Hydraulic properties of hydrogeologic units in the Tularosa Basin (after Bedinger, et.al, 1989).

The younger valley fill, on which the White Sands Dune field and Lake Lucero rest, has highly variable permeability due to complex mixtures of lithologies in the subsurface. Fine-grained clastic rocks (shales, silts) associated with lacustrine sediments, including those of the lower part of the Pleistocene Lake Otero sediments, have very low hydraulic conductivity. Thus water must flow downgradient toward the Hueco basin within sands intercalated with the shales.

The older consolidated rocks outcropping around the basin edge also conduct groundwater toward the basin, and serve as recharge areas, absorbing precipitation and runoff from the nearby mountains, where rain and snowfall are much greater. For example, average annual precipitation at Cloudcroft, in the mountains above Alamogordo, is 26.5 inches, whereas at White Sands National Monument it is only 9 inches.

Recharge also occurs through permeable fill, especially the alluvial fans, along the edge of the basin from freshwater streams running from the hills. The evidence for recharge (and dissolution of evaporites that are in turn carried toward White Sands) is quite dramatic in the basin, due to the shallow evaporites of either Pliestocene or Permian origin that have been dissolved by fresh water. Almendinger and Titus (1973) catalogued a number of these occurrences. For example Wier (1965) identified cavernous sections in the Torres member of the Yeso Formation in the subsurface of the northern Tularosa basin as a primary reservoir for domestic water in the area. Weber (1964) notes that surface water drains into solution cavities in gypsum and limestone on the west side of the Malpais lava flow. At the southern tip of the flow waters issuing from Malpais Spring have a high sodium cloride content (Na 3,555 ppm, Cl 13,000 ppm). Additionally, the basal beds of the Permian San Andres Formation where exposed in the Carrizozo quadrangle have random strikes and dips as a result of solution of underlying evaporites, with resulting subsidence and draping of undissoved rock. Almendinger notes that one section of the Malpais basalt flow has collapsed into a gypsum solution cavity, breaking through more than 150 feet of basalt. This is due mainly to the fact that the lava flow blocks runoff from the west, which ponds at the edge of the flow, then percolates downward, causing rapid dissolution of underlying gypsum. Herrick, (1904 p. 187) noted that the sand dunes on the eastern side of White Sands dam storm waters from the Sacramento mountians. Infiltration from the ponded waters accelerates the solution of underlying Quaternary lacustrine deposits, forming many sinkholes and caverns.

Precipitation and ground water flow regime:

Average precipitation follows patterns stiplulated by the mountains, with greatest rainfall by far in the Sacramento mountains ( 30 inches or more) on the eastern side of the basin versus in the San Andres mountains (14-16 inches) on the west. The highlands of the Chupadera Mesa in the northern basin extremity are also a prime recharge site, receiving 14 to 18 inches per year (See Chapter 9, Climate.). This basin-margin precipitation re-charges the groundwater aquifers as well as surface streams that flow into central parts of the Tularosa basin. There is also a seasonal pattern to precipitation, with a summer maximum in both the valley and mountains (Tables 3-2 and 3-3)

Table 3-2 Mean annual and monthly Precipitation: White Sands National Monument (Valley floor) Inches (1939-1999).

Annual mean precip: 8.97 inches

Table 3-3 Mean annual and monthly Precipitation: Cloudcroft (Sacramento Mountains) Inches (1914-1987)

Annual mean precip: 26.46 inches

The pattern of groundwater and surface flow in the basin is generally from the drainage divides in the surrounding uplands laterally into the valley, thence southward axially toward the Hueco basin (Figure 3-1). Figure 3-1 illustrates the relatively rapid movement of water into the basin from the Chupadero Mesa in the north, and the axial pattern of subsurface flow along the western, deepest side of the basin. Note also the surface discharge areas near Lumly Lake, Foster Lake and Lake Lucero near the White Sands dune field. In fact, the work of Almendinger and Titus reveals a water table depression of up to ten feet at Lake Lucero during the driest times of the year (Almendinger and Titus, 1973). The ultimate discharge of the system is the Rio Grand River to the south near El Paso. Because the Tularosa basin is so large, and groundwater slopes so shallow, the groundwater gradient south of the discharge areas on the eastern side of the basin at Salt Creek and Malpais Spring is only .0007. Locally, discharge also occurs from many small cold springs in the Sacramento Mountians and a few small cold springs in the San Andres Mountains (Bedinger, et. al, 1989). Discharge from sediments on the western side of the basin is almost exclusively to the Rio Grande.

Salinity of Groundwater:

In general, the pattern of water salinity in the basin is that of steady increase from freshwater in the mountains to hypersaline brines at lake Lucero and other evaporitive discharge points. Terminology for discussing salinity of waters is shown in Table 3-4.

Table 3-4 Terminology for water salinities, 1000's of mg/l dissolved solids Seawater is approx 35

The pattern of increasing salinity from the margins of the basin toward the center is evident in Figure 3-2. The fresh water lenses on the east and west sides of the basin are the result of the infiltration of fresh runoff into porous and permeable alluvial fans along the mountain fronts. Figures 3-3A and 3-3B illustrate this phenomenon for mountain-to-basin cross sections just south of Alamogordo and opposite Lake Lucero, respectively. It can be plainly seen that current runoff is contributing some dissolved solids, but the water coming from the mountains is essentially fresh. Using resistivity profiles Orr and Myers (1986) mapped freshwater lenses nearly 1000 feet thick in alluvial fans northward along the west side of the basin to Rhodes Canyon and beyond.Thus, most of the evaporites being concentrated at Lake Lucero and elsewhere on the Alkali flat are probably coming from more saline groundwaters migrating in the subsurface, mainly from the north, as suggested by Figure 3-1.

Seasonal Phenomena

Several factors in the climate and hydrology of the Tularosa basin cause strong seasonal phenomena to occur in the depositional system at the White Sands. These are (1) the widespread existence of a shallow groundwater table (2) seasonally high evaporation and evapotranspiration rates due to extreme heat and stronge winds (3) sudden influxes of fresh water into the saline groundwater system by flash floods and rainstorms, (Figures 3-4 (a-c) and 3-4 (d-f)). This controls and events can cause sudden and extreme changes in level of water table and salinity of groundwater, sometimes quite locally.

Throughout the area of the dunefield, groundwater table is relatively shallow - anywhere from the surface to a few feet below the surface. This has occurred because of the great volumes of water that have accumulated in the basin which is full essentially to the spill point after eons of inflow from nearby mountains. This also has occurred because to a certain extent the present level of the dunefield has been controlled by water table levels. Shallow ground water tables can affect eolian processes by limiting downcutting by wind. This occurs due to both capillary forces associated with damp sand, and more importantly by cementation that occurs when shallow waters evaporate and cementing minerals such as gypsum and halite are deposited near the surface (Schenk and Fryberger, 1988).

One seasonal phenomenon that occurs is the development of extremely evaporitic conditions on Lake Lucero, the surface of the Alkali flat and the interdunes of the dunefield due to high evaporation is summer. As a result, there are times of the year when only the toughest micro-organisms can survive in sediments near the surface. The summer evaporation can also draw down the water table locally, as noted by Crabaugh (in press) (Figure 3-5). Such hypersaline spikes may limit vegetation types that can survive in these settings.

On the other hand, there are other times that the near surface groundwater becomes unusually fresh. This is due to sudden rainfall and resultant flooding, which can inject large amounts of fresh water into the middle of the dunefield or onto Lake Lucero (Figures 3-4 (a-c) and 3-4 (d-f)). This occurs either through rainfall directly on the dunefield, or by means of freshwater flash floods. Table 3-5 shows some extreme rainfall months at White Sands. Of course, in addition to sudden rainstorms, the rainfall occurrence is seasonal at White Sands, and in the mountains as well, with a summer peak, as noted above, so there is an annual cyclicity to these events.

Another phenomenon of interest with respect to near surface waters is the difference between the salinity of the saturated groundwater table, and that of meteoric water perched in sand sheets and dunes. Loose sand such as that found in the dunes absorbs rainfall very quickly. Much of the water infiltrates downward to become part of the water table, as described above. Some, however remains in the sand as vadose water ("damp" sand). This water, because it has not acquired salts like the saline groundwater, is relatively fresh most of the time. Thus, sand sheets and dunes above the top of the capillary fringe of the saline groundwater table can support biota that are much less salt-tolerant than the interdunes and playa surfaces subject to extreme hypersalinity events.

Table 3-5 Maximum precipitation in a single month: White Sands Inches (1950-1987) (From Basabilvazo, et. al., 1994).

Maximum annual precipitation: 23.62 inches

List of Illustrations:

Figure 3-1. Map showing relative groundwater travel times and flow directions in the Tularosa Basin (after Bedinger, et. al., 1989). In fact, subsurface flows in the basin are very slow, and not necessarily throughgoing at all times. This condition perhaps coupled with a subtle subsurface drainage divide south of Lake Lucero is what enabled the formation of Lake Otero During the Pleistocene and later the evaporation of it's waters to form the gysiferous deposits of the final phase of dessication. Note surface discharge areas at Lumley Lake, Foster Lake and Lake Lucero within the basin and near the White Sands Dune field - a reflection of the shallowness of the groundwater table in this region.

Figure 3-2 Water quality zones and consolidated rock lithologies in the Tularosa Basin (after JS Mclean, 1970).

Figure 3-3 Freshwater lenses in alluval deposits along the Sacramento and San Andres mountains. (A) Water quality zones just south of Alamosa (B) Water quality zones near lake Lucero.

Figures 3-4 (a-c) and 3-4 (d-f) Seasonal floods of various kinds at White Sands. (A) Lake Lucero in flood following rains. This most commonly occurs during the summer rainfall peak (B) Drying of the shallow lake as the water evaporates. This photograph shows south Lake Lucero. "Lake" Lucero is more accurately referred to as a playa since the depositional system is that of an ephemeral, evaporitic playa than a perrenial lake. (C) Lost Creek in flood, bringing a sudden injection of fresh water into the dunefield at the terminus of the creek (D) Flooding of interdunes following winter rains. This is a time of low evaporation. The rainfall infiltrated rapidly through the dunes, and raised and freshened the water table locally. (E) Evidence of fresh water input through runoff from the mountains. Tongues of alluvial sand and gravel extend onto the playa surface at Lake Lucero. (F) View of the flat surface of the alkali flat, which is underlain, sometimes within a foot or two, by saline groundwater. The presence of the shallow groundwater table may regulate the scour at this surface through dampness and cementation by evaporites, thus regulating sand supply to the dunefield. A rise in water tableor return of full sabkha conditions such as those found at Lake Lucero would restrict sand supplies by immobilizing surface sediments further than they presently are.

Figure 3-5. (A) Drawdown of water table during a short term study within the White Sands Dune field, which seems to fit well the average long-term evaporation rate (after Crabaugh, in press).

Figure 3-6. Increase in salinity of ground water over a 60 day period at Lake Lucero for (A) Sodium ions and (B) Sulphate ions. After Almendinger and Titus, (1973).