In environmental science, inland freshwater bodies are defined as having salinity less than approximately how many parts per thousand of dissolved salts?

Introduction / Context:
Water bodies on Earth vary widely in their salt content or salinity. Freshwater, brackish water and marine water are classified largely based on this parameter. Inland water bodies such as lakes, rivers and ponds are usually considered freshwater, meaning that their salinity is very low compared with that of the oceans. This question asks you to choose the approximate upper limit of salinity, measured in parts per thousand, used to describe inland freshwater bodies.

Given Data / Assumptions:

- Salinity is measured in parts per thousand, often written as per mille or parts per thousand by mass.
- The focus is on inland freshwater bodies such as rivers and lakes.
- Options are 5, 20, 50 and 75 parts per thousand.
- We assume typical textbook definitions of freshwater versus saline water.

Concept / Approach:
Freshwater systems have very low salt content compared with sea water. Sea water salinity is around 35 parts per thousand on average. Freshwater is typically defined as water with salinity less than about 0.5 parts per thousand, which can also be expressed as less than 5 parts per thousand in some simplified classification tables aimed at easy memorisation. Higher values such as 20, 50 or 75 parts per thousand represent brackish or highly saline conditions rather than freshwater.

Step-by-Step Solution:
Step 1: Recall that sea water has salinity of around 35 parts per thousand.Step 2: Understand that freshwater must have much lower salinity than sea water.Step 3: Among the given options, the smallest value is 5 parts per thousand, which is consistent with a low salinity classification for freshwater in basic exam material.Step 4: Values of 20, 50 and 75 parts per thousand are much closer to or above sea water salinity and are therefore not suitable as upper limits for freshwater.Step 5: Conclude that inland freshwater bodies are defined as having salinity less than approximately 5 parts per thousand.

Verification / Alternative check:
You can verify by remembering that brackish water, such as that in estuaries, has moderate salinity values between freshwater and sea water, often above about 5 parts per thousand. Marine water values cluster around 35 parts per thousand. Since inland rivers and lakes are considered freshwater, their salinity needs to be below the threshold where water is treated as brackish. Therefore a value like 5 parts per thousand as a maximum fits this conceptual framework better than the higher options.

Why Other Options Are Wrong:
20 parts per thousand: This salinity is more characteristic of brackish water or partly saline water rather than freshwater.
50 parts per thousand: This value exceeds average sea water salinity and would represent very salty water, sometimes found in closed basin lakes, not typical freshwater.
75 parts per thousand: This extremely high salinity is found only in highly saline lakes or brine pools and is far from any reasonable freshwater classification.

Common Pitfalls:
Students may misread units and confuse percentages, parts per million and parts per thousand, leading to incorrect assumptions about how small the salinity of freshwater really is. Another issue is assuming that a larger number must be correct simply because it seems more impressive. Remember that freshwater systems have very low salt contents, so the smallest option is the most appropriate in this context. Linking the idea that sea water is around 35 parts per thousand helps you place all other values on a relative scale.

Final Answer:
Inland freshwater bodies have salinity less than approximately 5 parts per thousand.