water resources has focused on surface water or ground water as if they were separate entities. As development of land and water resources increases, it is apparent that development of either of these resources affects the quantity and quality of the other. Nearly all surface-water features (streams, lakes, reservoirs, wetlands, and estuaries) interact with ground water. These interactions take many forms. In many situations, surface-water bodies gain water and solutes from groundwater systems and in others the surface-water body is a source of groundwater recharge and causes changes in groundwater quality. As a result, withdrawal of water from streams can deplete ground water or conversely, pumpage of ground water can deplete water in streams, lakes, or wetlands. Pollution of surface water can cause degradation of groundwater quality and conversely pollution of ground water can degrade surface water. Thus, effective land and water management requires a clear understanding of the linkages between ground water and surface water as it applies to any given hydrologic setting. This Circular presents an overview of current understanding of the interaction of ground water and surface water, in terms of both quantity and quality, as applied to a variety of landscapes across the Nation. This Circular is a product of the GroundWater Resources Program of the U.S. Geological Survey. It serves as a general educational document rather than a report of new scientific findings. Its intent is to help other Federal, State, and local agencies build a firm scientific foundation for policies governing the management and protection of aquifers and watersheds. Effective policies and management practices must be built on a foundation that recognizes that surface water and ground water are simply two manifestations of a single integrated resource. It is our hope that this Circular will contribute to the use of such effective policies and management practices.

We present the Pristine survey, a new narrow-band photometric survey focused on the metallicity-sensitive Ca H & K lines and conducted in the northern hemisphere with the wide-field imager MegaCam on the Canada-France-Hawaii Telescope (CFHT). This paper reviews our overall survey strategy and discusses the data processing and metallicity calibration. Additionally we review the application of these data to the main aims of the survey, which are to gather a large sample of the most metal-poor stars in the Galaxy, to further characterise the faintest Milky Way satellites, and to map the (metal-poor) substructure in the Galactic halo. The current Pristine footprint comprises over 1,000 deg 2 in the Galactic halo ranging from b ∼ 30 • to ∼ 78 • and covers many known stellar substructures. We demonstrate that, for SDSS stellar objects, we can calibrate the photometry at the 0.02-magnitude level. The comparison with existing spectroscopic metallicities from SDSS/SEGUE and LAMOST shows that, when combined with SDSS broad-band g and i photometry, we can use the CaHK photometry to infer photometric metallicities with an accuracy of ∼0.2 dex from [Fe/H] = −0.5 down to the extremely metal-poor regime ([Fe/H] < −3.0). After the removal of various contaminants, we can efficiently select metal-poor stars and build a very complete sample with high purity. The success rate of uncovering [Fe/H] SEGUE < −3.0 stars among [Fe/H] Pristine < −3.0 selected stars is 24% and 85% of the remaining candidates are still very metal poor ([Fe/H]< −2.0). We further demonstrate that Pristine is well suited to identify the very rare and pristine Galactic stars with [Fe/H] < −4.0, which can teach us valuable lessons about the early Universe. . The observations at the Canada-France-Hawaii Telescope were performed with care and respect from the summit of Maunakea which is a significant cultural and historic site.†

FOREWORD oday, many concerns about the Nation's groundwater resources involve questions about their future sustainability. The sustainability of groundwater resources is a function of many factors, including depletion of groundwater storage, reductions in streamflow, potential loss of wetland and riparian ecosystems, land subsidence, saltwater intrusion, and changes in groundwater quality. Each groundwater system and development situation is unique and requires an analysis adjusted to the nature of the existing water issues. The purpose of this Circular is to illustrate the hydrologic, geologic, and ecological concepts that must be considered to assure the wise and sustainable use of our precious groundwater resources. The report is written for a wide audience of persons interested or involved in the protection and sustainable use of the Nation's water resources.

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