Dissolved Solids, Sediments, & Turbidity

What is TDS?

total concentration of dissolved substances in water

• Inorganic salts: sodium, potassium, calcium, magnesium, chlorides, sulfates, silicates, bicarbonates

• Small amounts of organic matter from natural sources

the "invisible" dissolved content

What TDS measures?

All dissolved ions and molecules

What does TDS not include?

suspended particles, bacteria, or anything that can be filtered out

TDS in freshwater? For drinking water?

20-1000 mg/L

50 mg/L for taste, 500 mg/L EPA recommended

is TDS unhealthy?

No

indirect indicator of contamination

How to measure TDS?

Directly: Evaporate a water sample, weigh
the remaining residue

Indirectly: Specific conductance (SpC) is a
proxy for TDS in freshwaters

Controls on TDS

• Soil type

• Geology/weathering

  • Ex. Limestone is relatively easy to mineralize

• pH/temperature/DO

• Discharge & Residence time

• Evaporation

• Decomposition & DOM

• Human Activities

Wentworth Scale

defines grain size class in intervals that
increase by powers of 2

How does particle size change from upstream to downstream?

Particle size tends to decrease
moving downstream

Median size (d50)

half of the bed particles are large, half are smaller

Embeddedness

degree to which particles are buried

Sorting

distribution of particles in the bed
• Well-sorted = discrete patches of the same size
• Poorly sorted = mixed particle sizes

Armoring

finer sediments overlain by larger particles
• Occurs when the bed surface of gravel-bed rivers is coarsened relative to the subsurface
• Dams can exacerbate this process

How do sediments move in streams?

Stream load – transport of matter other than water by streamflow

Dissolved load

mostly dissolved chemicals (TDS!), including
nutrients

Particulate load

non-dissolved components

Suspended (solid) load, wash load, bed load

Suspended (solid) load

material in the water column (<0.5 μm)

wash load

between 0.5 μm and 0.063 mm (largely sand and silt); think about the puff of sand when you step in a stream

bed load

material that rolls along the bottom (>0.063 mm)

shear stress

The amount of tractive force (“drag”) applied parallel to a surface
Will determine if a particle enters the stream load

highest in the “thalweg” (mainthread of flow)

Critical shear stress (τc)

shear stress necessary to mobilize
a given grain size

Shear increases

with both slope and depth

main driver of sediment transport?

storms

Entrainment (erosion)

getting the sediments moving

sand is most easily entrained

sediment transport (movement)

movement down the channel;
requires a lower velocity

Deposition (sedimentation)

settling out; as velocities decrease, grains settle out
Heaviest/largest sediments fall out first

Channel Modifications

erosion

accretion

sediment routing

Measuring sediments

• Total suspended solids (TSS) – filtering a water sample until filter clogs

• Direct turbidity measurement - beam of light is passed through a water sample; measure amount of light scattered (units = NTU)

  • ↑ scattered light with ↑ turbidity

• Secchi Disk – common in lakes