Some info on the subject. Your hydrologic cycle is short and through gravel and sand. Low organic content and high DO moving river water. Thus clear water with high Dos and low N. entering your pond.
What Factors Affect Ground-Water Flow and Chemistry?
Rates and pathways of ground-water flow, and the likelihood that ground water will be contaminated by nitrate, depend on a combination of factors. These factors include soil and aquifer composition, land use, and topography.
The grain size of soil and aquifer sediment affects the rates and pathways of ground-water flow. Coarse-grained sediment forms permeable aquifers through which water readily flows. Fine-grained sediment forms low-permeability aquifers (and confining units) that retard ground-water flow.
The amount of organic material in soil and aquifer sediment affects water chemistry. The amount of organic material generally is minimal in coarse-grained sediment and tends to be more abundant in fine-grained sediment. Organic material reacts with dissolved oxygen, removing it from the water. In areas where organic material is abundant, dissolved oxygen can become rapidly depleted.
Soil composition, topography, and land use commonly reflect the factors that affect ground-water flow and chemistry. Coarse-grained and well-drained soil that contains little organic material often indicates the presence of underlying coarse-grained aquifer sediment that also drains well and contains little organic material. Large upland agricultural fields are commonly located where these sediments are present. Fine-grained and poorly drained soil that contains abundant organic material often indicates the presence of underlying fine-grained aquifer sediment that also drains poorly and contains abundant organic material. Lowland forests and marshes and small fields interspersed in upland forests are commonly located in these areas unless extensive measures are used to drain the land.
The availability of dissolved oxygen and organic material affects the potential for nitrate contamination of ground water. Nitrogen is present in several forms in ground water, and it is transformed differently among these forms depending on the availability of dissolved oxygen and organic material. Where dissolved oxygen is abundant, bacteria chemically oxidize (nitrify) ammonia to nitrate. Nitrate is of concern because it is the one form of nitrogen that readily dissolves in, and flows with, ground water. Where dissolved oxygen is limited, nitrate will not form, and bacteria chemically reduce (denitrify) nitrate that is already present to nitrogen gas (the most common product), or to ammonia if sufficient organic material is present.
Ground-water age and chemistry
The age of the water ranged from 0 to 26 years and increased with depth and along pathways of ground-water flow (fig. 3). Thus, the chemistry of the oldest ground water reflects the effects of land use as early as 1968.
Dissolved-oxygen concentration ranged from 8.0 to less than 1.0 mg / L and decreased with depth and along ground-water flow paths. Nitrate concentration ranged from 7.7 to 27 mg / L as N in water beneath the agricultural field. The lowest nitrate concentration (0.13 mg / L) was in water in the fine-grained sediment.
Typically, underground
water has high CO2 concentrations,
and low pH and oxygen concentrations.
Carbon dioxide is
high in underground water because
of bacterial processes in the
soils and various underground,
particulate mineral formations
through which water moves. As
ground- or rainwaters flow over
and percolate through soil and underground
rock formations containing
calcitic limestone (CaCO3)
or dolomitic limestone
[CaMg(CO3)2], the acidity produced
by CO2 will dissolve limestone
and form calcium and magnesium
bicarbonate salt
Here is a link to a chapter on GW and basic geology.
http://www.dpla2.water.ca.gov/publications/groundwater/bulletin118/Bulletin118-Chapter6.pdf
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