Explanation of Quran 25:55 and 55:19-21

Question

In the Quran Allah says he released the two water source freshwater and salt water. Put barrier between them so they don't transgress.

He released the two seas, meeting [side by side]; (19)
Between them is a barrier [so] neither of them transgresses. (20)
So which of the favors of your Lord would you deny? (55:19-21)

Quran 25:55 and 55:19-21. Ibn KATHIR and bunch of other scholars explained the barrier is dry land that prevent the saltwater transgressing against freshwater. Meaning sea doesn't destroy the freshwater sources.

There is something call saltwater intrusion, it's due to sea lvl rising or strom or land eroding due to high tide or storm etc. (I am only referring to natural cause not human induced like climate change cause it etc.).

So how to understand the verse when it says they don't transgress but here we know naturally there are time when salt water intrusion happens, and it destroys the freshwater sources.

Also I looked into the explanation of salt water and fresh water don't mix due to saline lvl. This modern argument don't fit cuz given enough time it does mix.

I don't mean to cast any doubts, may Allah forgive me for if I said something wrong?

Answer 1

Introduction

The whole discussion and confusion in this verse revolves around the question: When the Quran says something does not happen, what does that mean?

So, when Allah says there is a barrier which neither of the two seas transgress, what is the implication of this statement? Does it mean the barrier is impenetrable and there is never an instance of the two seas meeting?

Or when Allah says in Surah Furqan that the two seas have a ⟪prohibiting partition⟫ between them, what is the implication of this? Does it mean the partition can never be violated or always exists?

All of this I would say is a misunderstanding of the language of the Quran and its intention.

Meaning of a Negation

It is not the intention of the Quran saying "X prevents something" when describing nature that it is an absolute prevention or an efficient cause for the prevention.

Nor does saying ⟪they do not transgress⟫ indicate "they never ever transgress."

That is all trying to read the Quran too much as a formula book describing facts about the world when the Quran is actually a book describing aspects of nature in very evocative and descriptive language.

If someone looked at the ocean where a river went into it and said, "Look at that, the seawater is not going into the freshwater!" it would be foolish for someone to say, "Actually, they do eventually mix."

That is because your point was not to claim they never go into each other. You were pointing out this phenomenon of them not going into each other in that place.

Likewise, when Allah says, ⟪Between them is a barrier that they do not cross⟫, why would anyone understand this as "They will never ever cross at all"??

Example

I will give you an obvious example in the Quran to prove this point, that a negation of something is not intended to indicate forever-complete-negation.

Allah says about the mountains ⟪We placed within the earth firmly set mountains, lest it should shift with them⟫

Leaving aside the exact tafsir of the relationship between mountains and earthquakes, this verse is clearly not meant to deny the existence of earthquakes!

Allah says He ⟪made the earth a stable ground⟫ and ⟪He made the earth tame for you⟫, all of these similarly negating the instability and movement of the earth, but that doesn't negate the existence of earthquakes!

Conclusion

As you can see, the negation of something can simply be a description of it in a particular time or place or a description of the majority or totality of it. So, saying the two seas do not transgress each other does not entail there will never be transgression at all. It can simply be a description of the phenomena when they actually do not transgress each other.

This applies to both of the famous interpretations of this verse, either referring to the rivers meeting the sea and subsequent separation of the two types of water before they are mixed completely or referring to land separating freshwater and seawater.

Answer 2

Three instances which mention a barrier

First of all, in the qur'an there are at least three instances where a barrier between salty seawater and freshwater of rivers etc. is mentioned:

1. And it is He who has released [simultaneously] the two seas, one fresh and sweet and one salty and bitter, and He placed between them a barrier and prohibiting partition. (25:53)

2. Is He [not best] who made the earth a stable ground and placed within it rivers and made for it firmly set mountains and placed between the two seas a barrier? Is there a deity with Allah ? [No], but most of them do not know. (27:61)

3. He released the two seas, meeting [side by side]; (19)
   Between them is a barrier [so] neither of them transgresses. (20)
   So which of the favors of your Lord would you deny? (55:19-21)

Ibn Kathir's interpretation in his tafsir

Since you've explicitly mentioned the tafsir of ibn Kahtir let me roughly address his explanation for each of the above instances:

In his explanation of instance #1 he really said that this barrier is a land barrier as you've stated (while mentioning this he also referred to instances #2 and #3) (see also in qtafsir).

As for instance #2 he mentioned a barrier without describing its nature, but he described the barrier itself as follows:

( وَجَعَلَ بَيْنَ الْبَحْرَيْنِ حَاجِزًا ) أَيْ : جَعَلَ بَيْنَ الْمِيَاهِ الْعَذْبَةِ وَالْمَالِحَةِ حَاجِزًا ، أَيْ : مَانِعًا يَمْنَعُهَا مِنَ الِاخْتِلَاطِ ، لِئَلَّا يَفْسُدَ هَذَا بِهَذَا وَهَذَا بِهَذَا ، فَإِنَّ الْحِكْمَةَ الْإِلَهِيَّةَ تَقْتَضِي بِقَاءَ كُلٍّ مِنْهُمَا عَلَى صِفَتِهِ الْمَقْصُودَةِ مِنْهُ ، فَإِنَّ الْبَحْرَ الْحُلْوَ هُوَ هَذِهِ الْأَنْهَارُ السَّارِحَةُ الْجَارِيَةُ بَيْنَ النَّاسِ . وَالْمَقْصُودُ مِنْهَا : أَنْ تَكُونَ عَذْبَةً زُلَالًا يُسْقَى الْحَيَوَانُ وَالنَّبَاتُ وَالثِّمَارُ مِنْهَا . وَالْبِحَارُ الْمَالِحَةُ هِيَ الْمُحِيطَةُ بِالْأَرْجَاءِ وَالْأَقْطَارِ مِنْ كُلِّ جَانِبٍ ، وَالْمَقْصُودُ مِنْهَا : أَنْ يَكُونَ مَاؤُهَا مِلْحًا أُجَاجًا ، لِئَلَّا يَفْسُدَ الْهَوَاءُ بِرِيحِهَا
means, He has placed a barrier between the fresh water and the salt water, to prevent them from mixing lest they corrupt one another. Divine wisdom dictates that each of them should stay as it is meant to be. The sweet water is that which flows in rivers among mankind, and it is meant to be fresh and palatable so that it may be used to water animals and plants and fruits. The salt water is that which surrounds the continents on all sides, and its water is meant to be salty and undrinkable lest the air be corrupted by its smell, (Source of the Translation: qtafsir)

Again in the 3rd instance, ibn Kathir claimed that the barrier is a land barrier.

An attempt to explain and consolidate

(See also islamqa fatwa #165094)
When we check books of tafsir we find several attempts to explain the barrier and several explanations of the nature of the water sources, some of which one can reject by today's knowledge.
Before going into deep we must understand Allah's creation as the wonder it is and don't take the qur'an as a science book that can give us explicit and correct scientific answers, we rather must read and try to interpret such potential scientific topics in the quranic text in the context of a layman's description of Allah's creation.
So let's try to understand the topic of the barrier: The main function of a barrier is to separate two things. For example, a wall can be a barrier, a wall has some thickness and separates two rooms creating a distance between them.
In our context, the quote above from ibn Kathir explains very well what the barrier between saltwater and freshwater should achieve.
This function can be achieved somehow by a land barrier, so that the freshwater of a river behind the land remains untouched, at least if we expect that this river and the sea are separated this should work pretty fine, but usually, as we know a river somehow ends in the sea, so this explanation doesn't seem to be enough for the general case.
And there could be an unseen barrier as also presented as one of two main explanations (solutions presented by scholars) in the fatwa mentioned above: Whether the barrier is small or big isn't mentioned the point is that at some place if you taste the water it is salty while if you move to the other side you may find fresh water, this could be a short distance or a long distance even if we expect that salty and freshwater may mix at some section, due to natural causes this section can be understood as the barrier itself or a part of it because at a distance from that you may either taste a (rather) salty or a (rather) fresh that can easily be potable for animals and fruits of the land and in the other side the water is only good for sea animals and plants. This second view was for example held by scholars such as al-Qurtubi and Tahar ibn 'Ashur in their tafsirs.