Playas may form in both hydrologically open and closed systems. Several previous classification schemes of playas rely heavily on geomorphic features rather than the groundwater hydrology of the system and the degree to which the basin is hydrologically open or closed. This paper presents a review of the literature on playa classifications and their hydrologic characteristics, which can be used to better define the hydrology of playas. A modification of the original hydrologically based classification schemes of Meinzer (1922) and Snyder (1962) illustrates the importance of groundwater in playa systems. In this paper a playa is defined as an intracontinental basin where the water balance of the lake (all sources of precipitation, surface-water flow, and groundwater flow minus evaporation and evapotranspiration) is negative for more than half the year, and the annual water balance is also negative. The playa surface must act as a local or regional discharge zone. Evidence of evaporite minerals will generally be present in parts of the basin. This includes carbonate minerals that can be demonstrated to have been formed through evaporative processes. A recharge playa is defined as above, except the playa surface acts as a means for recharging water to the aquifer. In this case, evaporite minerals are absent. Although evaporite formation is an important part of a playa sequence, significant accumulations of subaqueously deposited evaporites are only possible when saline water bodies are partially maintained by a constant inflow of groundwater. In hydrologically closed basins, the brine chemistry is influenced by the lithology of the sediments and bedrock within the playa catchment. In hydrologically open (through-flow) discharge complexes, the brine composition may be determined by the rate of groundwater flow through the basin (leakage ratio) relative to the weathering rate of the surrounding sediments and bed rock. If the groundwater inflow greatly exceeds the rate of chemical weathering, then the brine composition may be dominated by the chemical composition of regional or local precipitation. Extensive accumulations of subaqueous evaporites form in playas when there is sufficient groundwater to maintain a shallow brine pond for an extended period of time. This has important implications for paleoclimate reconstructions of closed basin playa sequences. Extremely arid periods, when there is insufficient water to maintain a brine, will result in displacive intrasediment growth of evaporites; and extremely wet periods may be too fresh for a brine to develop. It is only in the intermediate periods when evaporation is high and water input is balanced by evaporation that extensive subaqueous evaporites will accumulate in closed basin playas.

Microplastics are an environmental contaminant of growing concern, but there is a lack of information about microplastic distribution, persistence, availability, and biological uptake in freshwater systems. This is especially true for large river systems like the Colorado River that spans multiple states through mostly rural and agricultural land use. This study characterized the quantity and morphology of microplastics in different environmental compartments in two large reservoirs along the Colorado River: Lakes Mead and Mohave, within Lake Mead National Recreation Area. To assess microplastic occurrence, surface water and surficial sediment were sampled at a total of nine locations. Sampling locations targeted different sub-basins with varying levels of anthropogenic impact. Las Vegas Wash, a tributary which delivers treated wastewater to Lake Mead, was also sampled. A sediment core (33 cm long, representing approximately 19 years) was extracted from Las Vegas Bay to assess changes in microplastic deposition over time. Striped bass (Morone saxatilis), common carp (Cyprinus carpio), quagga mussels (Dreissena bugensis), and Asian clams (Corbicula fluminea) were sampled at a subset of locations to assess biological uptake of microplastics. Microplastic concentrations were 0.44–9.7 particles/cubic meter at the water surface and 87.5–1,010 particles/kilogram dry weight (kg dw) at the sediment surface. Sediment core concentrations were 220–2,040 particles/kg dw, with no clear increasing or decreasing trend over time. Shellfish microplastic concentrations ranged from 2.7–105 particles/organism, and fish concentrations ranged from 0–19 particles/organism. Fibers were the most abundant particle type found in all sample types. Although sample numbers are small, microplastic concentrations appear to be higher in areas of greater anthropogenic impact. Results from this study improve our understanding of the occurrence and biological uptake of microplastics in Lake Mead National Recreation Area, and help fill existing knowledge gaps on microplastics in freshwater environments in the southwestern U.S.

Ohaaki Pool was the main hot spring in the Broadlands-Ohaaki geothermal area before recent anthropogenic modification. The alkaline Na-HCO 3 -Cl water, which discharged at 95 degrees C with a flow rate of 10 l. (super -1) , flowed down a broad discharge apron into the Waikato River. The discharge apron was inhabited by thriving microbial communities that included Calothrix, Phormidium, and numerous Synechococcus. These microbes mediated the construction of columnar stromatolites around the edge of Ohaaki Pool, oncoids in the discharge channel, and intercalated stratiform stromatolites, "Conophyton", and coccoid microbial mats on the distal part of the discharge apron. All the microbes were variably replaced and encrusted by amorphous silica while alive or shortly after death. Consequently, fabrics in the siliceous sinters around Ohaaki Pool are controlled by the growth patterns and composition of the microbial community. For example, the Calothrix-dominated community gave rise to stratiform stromatolites that are characterized by alternating erect and prostrate laminae. Conversely, the Synechococcus-dominated communities formed mats that produced dense, white siliceous laminae. The Phormidium-Synechococcus community constructed the "Conophyton". Platy, skeletal, and blocky calcite crystals are found in, around, and between the siliceous stromatolites that formed around Ohaaki Pool, the proximal part of the discharge apron, and in the oncoids. Although minor calcite is found locally in the stratiform stromatolites on the discharge apron, there is no evidence that microbes played any role in calcite precipitation.

At high altitudes (>1000 m) throughout the Dry Valleys, Antarctica, liquid water is rare, yet ground ice, which may be millions of years old, is pervasive in glacial sediments and bedrock. The origin of this ice is different from its arctic and alpine counterparts and may be similar to water on Mars. We present chemical and isotopic analyses of Antarctic ground ice from cores of Sirius Group sediments at Table Mountain in the Dry Valleys. These data, together with the presence of diagenetic calcite and chabazite in the frozen sediments, indicate that the ice and minerals accumulated over long periods of time from atmospheric water vapor and brine films formed on the surface of the ground. This analogy indicates that ferric-bearing minerals could precipitate under present conditions in the Martian soil.