Coastal springs in northern Guam
Almost all springs in northern Guam are located along the coast. Only a few inland springs exist on the plateau, but their water issues from smaller bodies of rock (associated with volcanic terrain of Mt. Santa Rosa and Mataguac Hill and clay-rich limestone bordering on southern volcanic terrain) and does not constitute discharge from the Northern Guam Lens Aquifer. The only discharge features from the NGLA are coastal springs that dot the perimeter of northern Guam. They are classified based on their geomorphology into four categories:
1. Beach seeps and springs, which occur in the intertidal zone of sandy beaches. They are widespread and easily observed on practically any northern Guam beach. They are best seen during low tide and calm seas, when meandering channels, parallel rills, and meter-scale mini-deltas develop in the beach sand by the effluent. At high tide, most beach seeps and springs become submerged and are difficult to identify. The larger springs, however, remain visible even at high tides as shallow-water boils and some are made apparent by teeming growth of green algae (Fig. 5e) promoted by localized anthropogenic nutrient input. Seeps and springs are most prolific within sandy embayments bounded by rocky coast (Tumon, Hågatña, Haputo, and Tanguisson) and are less common on linear beaches (Falcona, Uruno, Ritidian, and Jinapsan).
2. Reef seeps and springs occur on the flats and along margins of fringing coral-algal reefs, in depths ranging from a few centimetres to a few meters. Some discharge points can be noticed in small openings on the reef, but in general, discharge is too diffuse for specific sources to be located. Nevertheless, in shallow areas that are protected by reefs and thus relatively warm and calm, the presence of fresh water plumes is obvious to snorkelers who can easily identify them by the distinct change in water temperature and unmistakable optical “blurring” due to refraction from density currents. Such plumes have been observed on the reef flats at numerous locations in Tumon Bay and at least one location in Hågatña Bay.
3. Fracture springs are the characteristic discharge feature in the parts of coastline occupied by rocky outcrops and sheer cliffs. They occur where joints and faults acting as preferential pathways for fresh water intersect the coast, and discharge the flow they conduct. Numerous fractures of various sizes extend perpendicular to scarps and coastlines in northern Guam and are exposed in the faces of sea cliffs and coastal outcrops. When seen from the coast, they appear as vertical and sub-vertical slits in the cliffs, extending up into the rock above as well as down below the ocean surface. Tracing the exposed part of a fracture downward along the rock face often reveals a small spring at the waterline. This phenomenon can be observed on a variety of scales, ranging from open joints only a few millimeters across to dissolution-widened fractures up to several meters wide, which accommodate entry and exploration by people. The former may emit mere trickles, whereas the latter discharge free-surface streams of fresh water flowing over a base of underlying seawater.
4. Caves springs, like fracture springs, are a characteristic discharge feature in the rocky parts of the northern Guam coast. They occur on various scales and in distinct morphologies, ranging in size and appearance from small interstices to dm-scale openings and large coastal caverns. Located at or near the sea level, they discharge fresh water from dissolutional cavities. They are not to be confused with pseudokarst caves (sea caves or littoral caves) produced in exposed coastal cliffs by mechanical erosion. Mechanically-gouged caves are frequent in straight stretches of wave-battered cliffline, whereas karst cave springs that discharge fresh water occur mostly in places where the cliffline has locally receded into coastal alcoves, in which the mixing of cool fresh water with the warm ocean water can be sensed when the sea is calm.
The first two categories of coastal springs are associated with matrix porosity and diffuse flow within the aquifer (or are fed by fissures buried under sediment). Concentrated discharges from fractures and caves are associated with fracture and karst conduit porosity.
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